Proving and Applying Polynomial Identities
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Algebra 2 › Proving and Applying Polynomial Identities
Generate a Pythagorean triple using the identity $$ (x^2 + y^2)^2 = (x^2 - y^2)^2 + (2xy)^2 $$ with $x=4$ and $y=1$. Which triple $(a,b,c)$ is correct (order of $a,b$ may be swapped)?
$(8,15,17)$
$(15,17,8)$
$(8,15,16)$
$(7,16,17)$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. The Pythagorean triple identity (x² + y²)² = (x² - y²)² + (2xy)² is remarkably powerful: it generates right triangles! If you pick any two integers x and y (with x > y), this identity gives you three expressions—x² + y², x² - y², and 2xy—that form a Pythagorean triple. With x = 4 and y = 1: First calculate a = x² - y² = 16 - 1 = 15, b = 2xy = 2(4)(1) = 8, and c = x² + y² = 16 + 1 = 17. Let's verify this is a Pythagorean triple: a² + b² = 15² + 8² = 225 + 64 = 289 = 17² = c². Perfect! The triple is (15, 8, 17), or equivalently (8, 15, 17) since order of a and b can be swapped. Choice A correctly identifies (8, 15, 17) as the triple. Choice B gives (7, 16, 17), but 7² + 16² = 49 + 256 = 305 ≠ 289 = 17². Choice D gives (8, 15, 16), using 16 instead of 17 for the hypotenuse. For generating Pythagorean triples with (x² + y²)² = (x² - y²)² + (2xy)²: (1) Pick two positive integers x and y with x greater than y, (2) Calculate the three quantities: a = x² - y², b = 2xy, c = x² + y², (3) Verify: a² + b² should equal c². Try x = 5, y = 2 next: you'll get the (21, 20, 29) triple!
Prove the identity $$ (a-b)^3=a^3-3a^2b+3ab^2-b^3 $$ by expanding the left-hand side. Which expression is the correct expansion of $(a-b)^3$?
$a^3-a^2b+ab^2-b^3$
$a^3+3a^2b+3ab^2+b^3$
$a^3-3a^2b-3ab^2-b^3$
$a^3-3a^2b+3ab^2-b^3$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. For example, $(a + b)^2 = a^2 + 2ab + b^2$ works whether a = 5 and b = 3, or a = -2 and b = 7, or any other values. To prove an identity, we show the two sides are algebraically equivalent by expanding, factoring, or manipulating until they match. Expand $(a - b)^3 = (a - b)(a - b)(a - b) = $ first $(a - b)^2 = a^2 - 2ab + b^2$, then times $(a - b) = a(a^2 - 2ab + b^2) - b(a^2 - 2ab + b^2) = a^3 - 2a^2b + ab^2 - a^2b + 2ab^2 - b^3 = a^3 - 3a^2b + 3ab^2 - b^3$. Choice B correctly expands to $a^3 - 3a^2b + 3ab^2 - b^3$. A distractor like Choice A might flip all signs incorrectly, but track the negative from $-b$ carefully. To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side. Alternatively, expand BOTH sides independently and show they give the same result. Either way, the goal is demonstrating the two expressions are algebraically equivalent for all variable values. Testing with specific numbers can give confidence but doesn't prove—you need algebraic demonstration!
Demonstrate that the polynomial identity $$ (x^2+y^2)^2=(x^2-y^2)^2+(2xy)^2 $$ holds for all real $x,y$ by expanding both sides and simplifying.
LHS: $(x^2+y^2)^2=x^4+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=x^4+y^4$, so the identity is true.
LHS: $(x^2+y^2)^2=x^4+2x^2y^2+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=(x^4+2x^2y^2+y^4)+4xy=x^4+2x^2y^2+y^4+4xy$ so they are equal.
LHS: $(x^2+y^2)^2=x^4+2x^2y^2+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=(x^4-2x^2y^2+y^4)+4x^2y^2=x^4+2x^2y^2+y^4$ so they are equal for all $x,y$.
LHS: $(x^2+y^2)^2=x^4+2x^2y^2+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=(x^4-2x^2y^2+y^4)+2x^2y^2=x^4+y^4$ so they match.
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. For example, (a + b)² = a² + 2ab + b² works whether a = 5 and b = 3, or a = -2 and b = 7, or any other values. To prove an identity, we show the two sides are algebraically equivalent by expanding, factoring, or manipulating until they match. Let's expand the left side: (x² + y²)² = x⁴ + 2x²y² + y⁴. Now the right side: (x² - y²)² + (2xy)² = (x⁴ - 2x²y² + y⁴) + 4x²y² = x⁴ - 2x²y² + y⁴ + 4x²y² = x⁴ + 2x²y² + y⁴. Choice B correctly shows both expansions and demonstrates they're equal, proving the identity holds for all x, y. Choice A makes an error in expanding (2xy)² as 2x²y² instead of 4x²y², while C incorrectly expands the squared binomials, and D adds an extra 4xy term that shouldn't be there. To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side. Alternatively, expand BOTH sides independently and show they give the same result. Either way, the goal is demonstrating the two expressions are algebraically equivalent for all variable values.
Apply the sum of cubes identity to factor completely: $$x^3 + 8.$$
$(x+2)(x^2-2x+4)$
$(x-2)(x^2+2x+4)$
$(x+8)(x^2-8x+64)$
$(x+2)(x^2+2x+4)$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. For example, $(a + b)^2 = a^2 + 2ab + b^2$ works whether $a = 5$ and $b = 3$, or $a = -2$ and $b = 7$, or any other values. To prove an identity, we show the two sides are algebraically equivalent by expanding, factoring, or manipulating until they match. For $x^3 + 8 = x^3 + 2^3$, apply the sum of cubes: $a^3 + b^3 = (a + b)(a^2 - ab + b^2)$, so $(x + 2)(x^2 - 2x + 4)$, and it's completely factored since $x^2 - 2x + 4$ has no real roots. Choice C correctly applies the identity to factor as $(x + 2)(x^2 - 2x + 4)$. A distractor like Choice A might use $+2x$ instead of $-2x$, but remember the middle term in sum of cubes is negative. To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side. Alternatively, expand BOTH sides independently and show they give the same result. Either way, the goal is demonstrating the two expressions are algebraically equivalent for all variable values. Testing with specific numbers can give confidence but doesn't prove—you need algebraic demonstration!
Verify that the polynomial identity $$a^3+b^3=(a+b)(a^2-ab+b^2)$$ is true for all real numbers $a$ and $b$ by expanding the right-hand side.
$$(a+b)(a^2-ab+b^2)=a^3-a^2b+ab^2+b^3=a^3+b^3-ab(a-b)$$
$$(a+b)(a^2-ab+b^2)=a^3+3a^2b+3ab^2+b^3$$
$$(a+b)(a^2-ab+b^2)=a^3-a^2b+ab^2+a^2b-ab^2+b^3=a^3+b^3$$
$$(a+b)(a^2-ab+b^2)=a^3-ab^2+a^2b-b^3=a^3-b^3$$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. For example, $(a + b)^2 = a^2 + 2ab + b^2$ works whether $a = 5$ and $b = 3$, or $a = -2$ and $b = 7$, or any other values. To prove an identity, we show the two sides are algebraically equivalent by expanding, factoring, or manipulating until they match. Expanding the right side: $(a + b)(a^2 - ab + b^2) = a \cdot a^2 + a \cdot(-ab) + a \cdot b^2 + b \cdot a^2 + b \cdot(-ab) + b \cdot b^2 = a^3 - a^2 b + a b^2 + a^2 b - a b^2 + b^3$, where $-a^2 b + a^2 b$ and $a b^2 - a b^2$ cancel, leaving $a^3 + b^3$. Choice A correctly proves the identity by showing the expansion and cancellation to match $a^3 + b^3$. To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side.
Demonstrate that the polynomial identity $$ (x^2+y^2)^2=(x^2-y^2)^2+(2xy)^2 $$ holds for all real $x,y$ by expanding both sides and simplifying.
LHS: $(x^2+y^2)^2=x^4+2x^2y^2+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=(x^4+2x^2y^2+y^4)+4xy=x^4+2x^2y^2+y^4+4xy$ so they are equal.
LHS: $(x^2+y^2)^2=x^4+2x^2y^2+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=(x^4-2x^2y^2+y^4)+4x^2y^2=x^4+2x^2y^2+y^4$ so they are equal for all $x,y$.
LHS: $(x^2+y^2)^2=x^4+2x^2y^2+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=(x^4-2x^2y^2+y^4)+2x^2y^2=x^4+y^4$ so they match.
LHS: $(x^2+y^2)^2=x^4+y^4$ and RHS: $(x^2-y^2)^2+(2xy)^2=x^4+y^4$, so the identity is true.
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. For example, (a + b)² = a² + 2ab + b² works whether a = 5 and b = 3, or a = -2 and b = 7, or any other values. To prove an identity, we show the two sides are algebraically equivalent by expanding, factoring, or manipulating until they match. Let's expand the left side: (x² + y²)² = x⁴ + 2x²y² + y⁴. Now the right side: (x² - y²)² + (2xy)² = (x⁴ - 2x²y² + y⁴) + 4x²y² = x⁴ - 2x²y² + y⁴ + 4x²y² = x⁴ + 2x²y² + y⁴. Choice B correctly shows both expansions and demonstrates they're equal, proving the identity holds for all x, y. Choice A makes an error in expanding (2xy)² as 2x²y² instead of 4x²y², while C incorrectly expands the squared binomials, and D adds an extra 4xy term that shouldn't be there. To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side. Alternatively, expand BOTH sides independently and show they give the same result. Either way, the goal is demonstrating the two expressions are algebraically equivalent for all variable values.
Use the identity $$ (x^2+y^2)^2=(x^2-y^2)^2+(2xy)^2 $$ to generate a Pythagorean triple when $x=3$ and $y=2$. (A Pythagorean triple is $(a,b,c)$ with $a^2+b^2=c^2$.)
$(13,12,5)$
$(12,5,13)$
$(6,5,13)$
$(5,12,13)$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. The Pythagorean triple identity (x² + y²)² = (x² - y²)² + (2xy)² is remarkably powerful: it generates right triangles! If you pick any two integers x and y (with x > y), this identity gives you three expressions—x² + y², x² - y², and 2xy—that form a Pythagorean triple. For x=3, y=2: a = 9 - 4 = 5, b = 2·3·2 = 12, c = 9 + 4 = 13, and 5² + 12² = 25 + 144 = 169 = 13². Choice A correctly generates the triple (5,12,13) using the identity. Other choices rearrange or alter the values incorrectly. For generating Pythagorean triples with (x² + y²)² = (x² - y²)² + (2xy)²: (1) Pick two positive integers x and y with x greater than y, (2) Calculate the three quantities: a = x² - y², b = 2xy, c = x² + y², (3) Verify: a² + b² should equal c².
Recognize and factor completely using polynomial identities: $$x^4 - 16.$$
$(x^2 - 4)(x^2 + 4)$
$(x - 4)(x^3 + 4)$
$(x - 2)(x + 2)(x^2 + 4)$
$(x - 2)^2(x + 2)^2$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. For example, $(a + b)^2 = a^2 + 2ab + b^2$ works whether a = 5 and b = 3, or a = -2 and b = 7, or any other values. To prove an identity, we show the two sides are algebraically equivalent by expanding, factoring, or manipulating until they match. For $$x^4 - 16 = (x^2)^2 - 4^2 = (x^2 - 4)(x^2 + 4) = (x - 2)(x + 2)(x^2 + 4)$$, using difference of squares twice for complete factorization. Choice C correctly factors completely as $(x - 2)(x + 2)(x^2 + 4)$. A distractor like Choice A stops at partial factorization, but complete means breaking down all factorable parts. To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side. Alternatively, expand BOTH sides independently and show they give the same result. Either way, the goal is demonstrating the two expressions are algebraically equivalent for all variable values. Testing with specific numbers can give confidence but doesn't prove—you need algebraic demonstration!
Prove the identity $$ (a - b)^3 = a^3 - 3a^2 b + 3 a b^2 - b^3 $$ by expanding $(a - b)^3$, then apply it to expand $(x - 2)^3$.
$ (x - 2)^3 = x^3 - 6x^2 + 4x - 8 $
$ (x - 2)^3 = x^3 - 6x^2 + 12x - 8 $
$ (x - 2)^3 = x^3 - 6x^2 - 12x - 8 $
$ (x - 2)^3 = x^3 - 4x^2 + 12x - 8 $
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. The binomial cube identity $$ (a - b)^3 = a^3 - 3a^2 b + 3 a b^2 - b^3 $$ follows a specific pattern with coefficients 1, -3, 3, -1. Let's apply it to $$ (x - 2)^3 $$ with a = x and b = 2: $$ (x - 2)^3 = x^3 - 3(x^2)(2) + 3(x)(2^2) - 2^3 = x^3 - 6x^2 + 3x(4) - 8 = x^3 - 6x^2 + 12x - 8 $$. Choice A correctly shows this expansion with all terms properly calculated. Choice B has 4x instead of 12x (forgetting to multiply 3 × 4), Choice C has -4x^2 instead of -6x^2 (using 2^2 instead of 3 × 2), and Choice D incorrectly has -12x instead of +12x (wrong sign pattern). To expand binomial cubes: (1) Remember the coefficient pattern: 1, 3, 3, 1 for both $$ (a + b)^3 $$ and $$ (a - b)^3 $$, (2) For $$ (a - b)^3 $$, signs alternate: +, -, +, -, (3) Powers decrease for a (from 3 to 0) and increase for b (from 0 to 3), (4) Calculate each term carefully—it's easy to make arithmetic errors with the coefficients!
Demonstrate that the identity $$a^2-b^2=(a+b)(a-b)$$ holds for all real $a,b$ by expanding the right-hand side. Which expansion is correct?
$(a+b)(a-b)=a^2-ab-b^2$
$(a+b)(a-b)=a^2+2ab-b^2$
$(a+b)(a-b)=a^2+b^2$
$(a+b)(a-b)=a^2-b^2$
Explanation
This question tests your understanding of polynomial identities—equations that are true for all values of the variables—and how to prove them algebraically or use them for applications like generating Pythagorean triples. A polynomial identity is different from a regular equation: it's true for ALL possible values of the variables, not just specific solutions. The difference of squares identity a² - b² = (a + b)(a - b) is one of the most useful factoring patterns in algebra. Let's expand the right side to verify: (a + b)(a - b) = a(a - b) + b(a - b) = a² - ab + ba - b² = a² - ab + ab - b² = a² - b². The cross terms -ab and +ab cancel perfectly! Choice A correctly shows that (a + b)(a - b) = a² - b². Choice B incorrectly includes a 2ab term, confusing this with the perfect square expansion. Choice C has -ab without the canceling +ab term. Choice D claims the result is a² + b², which would be the sum of squares, not the difference! To prove a polynomial identity: (1) Pick the more complicated side (usually), (2) Expand using FOIL, distribution, or combining like terms, (3) Simplify systematically, (4) Show it equals the other side. The difference of squares pattern is special because the middle terms always cancel, leaving just a² - b²!