PSAT Math : How to use FOIL

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Use Foil

Factor 2x2 - 5x – 12

Possible Answers:

(x - 4) (2x + 3)

(x + 4) (2x + 3)

(x + 4) (2x + 3)

(x – 4) (2x – 3)

Correct answer:

(x - 4) (2x + 3)

Explanation:

Via the FOIL method, we can attest that x(2x) + x(3) + –4(2x) + –4(3) = 2x2 – 5x – 12.

Example Question #1 : Exponents And The Distributive Property

x > 0.

Quantity A: (x+3)(x-5)(x)

Quantity B: (x-3)(x-1)(x+3)

Possible Answers:

Quantity B is greater

The relationship cannot be determined from the information given

The two quantities are equal

Quantity A is greater

Correct answer:

Quantity B is greater

Explanation:

 

Use FOIL: 

 

  (x+3)(x-5)(x) = (x2 - 5x + 3x - 15)(x) = x3 - 5x2 + 3x2 - 15x = x3 - 2x2 - 15x for A.

 

  (x-3)(x-1)(x+3) = (x-3)(x+3)(x-1) = (x2 + 3x - 3x - 9)(x-1) = (x2 - 9)(x-1)

  (x2 - 9)(x-1) = x3 - x2 - 9x + 9 for B. 

The difference between A and B: 

 (x3 - 2x2 - 15x) - (x3 - x2 - 9x + 9) = x3 - 2x2 - 15x - x3 + x2 + 9x - 9

 = - x2 - 4x - 9. Since all of the terms are negative and x > 0:

  A - B < 0.

Rearrange A - B < 0:

  A < B

 

 

 

Example Question #1 : Exponents And The Distributive Property

Solve for all real values of \(\displaystyle \small x\).

\(\displaystyle x^3+5x^2-10x=2x^2\)

Possible Answers:

\(\displaystyle 0,\ 2,\ -5\)

\(\displaystyle 2,\ 5\)

\(\displaystyle 2,\ -5\)

\(\displaystyle 0,\ 2,\ 5\)

Correct answer:

\(\displaystyle 0,\ 2,\ -5\)

Explanation:

\(\displaystyle x^3+5x^2-10=2x^2\)

First, move all terms to one side of the equation to set them equal to zero.

\(\displaystyle x^3+5x^2-2x^2-10x=0\)

\(\displaystyle x^3+3x^2-10x=0\)

All terms contain an \(\displaystyle \small x\), so we can factor it out of the equation.

\(\displaystyle x(x^2+3x-10)=0\)

Now, we can factor the quadratic in parenthesis. We need two numbers that add to \(\displaystyle \small 3\) and multiply to \(\displaystyle \small -10\).

\(\displaystyle -2*5=-10\ \text{and}\ -2+5=3\)

\(\displaystyle x(x-2)(x+5)=0\)

We now have three terms that multiply to equal zero. One of these terms must equal zero in order for the product to be zero.

\(\displaystyle \begin{matrix} x=0 & x-2=0 &x+5=0 \\ x=0 & x=2 & x=-5 \end{matrix}\)

Our answer will be \(\displaystyle \small x=0,2,-5\).

Example Question #4 : How To Use Foil

Simplify: \(\displaystyle (2x+3)^{2}\)

Possible Answers:

\(\displaystyle 4x^{2}+9\)

\(\displaystyle 4x^{2}+6x+9\)

\(\displaystyle 4x^{2}-9\)

\(\displaystyle 5x^{2}\)

\(\displaystyle 4x^{2}+12x+9\)

Correct answer:

\(\displaystyle 4x^{2}+12x+9\)

Explanation:

In order to simplify this expression, you need to use the FOIL method. First rewrite the expression to look like this: \(\displaystyle (2x+3)(2x+3)\)

Next, multiply your first terms together: \(\displaystyle 2x\times2x =4x^{2}\)

Then, multiply your outside terms together: \(\displaystyle 2x\times3=6x\)

Then, multiply your inside terms together: \(\displaystyle 3\times2x=6x\)

Lastly, multiply your last terms together: \(\displaystyle 3\times3=9\)

Together, you have \(\displaystyle 4x^{2}+6x+6x+9\)

You can combine your like terms, \(\displaystyle 6x+6x\), to give you the final answer: \(\displaystyle 4x^{2}+12x+9\)

Example Question #5 : How To Use Foil

Use FOIL to simplify the following product:

\(\displaystyle (x+9)(x-7)\)

Possible Answers:

\(\displaystyle x^2-63\)

\(\displaystyle x^2+11x+63\)

\(\displaystyle 7x^2-9x\)

\(\displaystyle x^2+2x-63\)

\(\displaystyle x^2+7x+54\)

Correct answer:

\(\displaystyle x^2+2x-63\)

Explanation:

Use the FOIL method (first, outside, inside, last) to find the product of:

\(\displaystyle (x+9)(x-7)\)

First: \(\displaystyle x\cdot x=x^2\)

Outside: \(\displaystyle -7\cdot x=-7x\)

Inside: \(\displaystyle 9\cdot x=9x\)

Last: \(\displaystyle -7\cdot 9=-63\)

Sum the products to find the polynomial:

\(\displaystyle x^2-7x+9x-63\)

\(\displaystyle x^2+2x-63\)

Example Question #6 : How To Use Foil

Simplify:

\(\displaystyle 2(x^{2}+x)(x-1)\)

Possible Answers:

\(\displaystyle 2x^{3} -2x\)

\(\displaystyle 2x^{3}+2x^{2}-2x-1\)

\(\displaystyle 2x^{3}+4x^{2}-2x\)

\(\displaystyle 2x^{3} -x\)

\(\displaystyle 2x^{3}+2x^{2}-2x\)

Correct answer:

\(\displaystyle 2x^{3} -2x\)

Explanation:

To solve this problem, use the FOIL method. Start by multiplying the First term in each set of parentheses: \(\displaystyle x^{2} \cdot x =x^{3}\) 

Then multiply the outside terms: \(\displaystyle x^{2}\cdot (-1) =-x^{2}\)

Next, multiply the inside terms: \(\displaystyle x \cdot x= x^{2}\)

Finally, multiply the last terms: \(\displaystyle x\cdot (-1) = -x\)

When you put the pieces together, you have \(\displaystyle 2(x^{3} -x^{2} +x^{2}-x)\). Notice that the middle terms cancel each other out, and you are left with \(\displaystyle 2(x^{3} -x)\). When you distribute the two, you reach the answer: \(\displaystyle 2x^{3} -2x\)

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