Precalculus : Pre-Calculus

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #1 : Determine The Symmetry Of An Equation

Is the following function symmetrical about the y axis (is it an even function)?

\(\displaystyle y=x^3+5x-7\)

Possible Answers:

Not a function

Yes

Insufficient Information

No

Correct answer:

No

Explanation:

For a function to be even, it must satisfy the equality \(\displaystyle f(x)=f(-x)\)

Likewise if a function is even, it is symmetrical about the y-axis \(\displaystyle f(-x)=(-x)^3+5(-x)-7=-x^3-5x-7\neq f(x)\)

Therefore, the function is not even, and so the answer is No

Example Question #1 : Determine The Symmetry Of An Equation

Algebraically check for symmetry with respect to the x-axis, y axis, and the origin.

\(\displaystyle x=-y^2+4\)

Possible Answers:

Symmetrical about the y-axis

Symmetrical about the origin

No symmetry

Symmetrical about the x-axis

Correct answer:

Symmetrical about the x-axis

Explanation:

For a function to be symmetrical about the y-axis, it must satisfy \(\displaystyle f(x)=f(-x)\) \(\displaystyle f(-x)=\sqrt{4+x}\neq f(x)\) so there is not symmetry about the y-axis

For a function to be symmetrical about the x-axis, it must satisfy \(\displaystyle f(-x)=-f(x)\) \(\displaystyle f(-x)=\sqrt{4+x}=-f(x)\) so there is symmetry about the x-axis

For a function to be symmetrical about the origin, you must replace y with (-y) and x with (-x) and the resulting function must be equal to the original function.

\(\displaystyle (-x)=-(-y)^2+4\neq x=-y^2+4\)

 

-So there is no symmetry about the origin, and the answer is Symmetrical about the x-axis

Example Question #1 : Determine The Symmetry Of An Equation

Algebraically check for symmetry with respect to the x-axis, y axis, and the origin.

\(\displaystyle y=|x|+2\)

Possible Answers:

Symmetry about the x-axis, and y-axis

Symmetry about the y-axis and origin

Symmetry about the y-axis 

Symmetry about the x-axis, y-axis, and origin

Symmetry about the x-axis

Correct answer:

Symmetry about the y-axis 

Explanation:

For a function to be symmetrical about the y-axis, it must satisfy \(\displaystyle f(x)=f(-x) f(-x)=|-x|+2=|x|+2\) so there is symmetry about the y-axis

For a function to be symmetrical about the x-axis, it must satisfy \(\displaystyle f(-x)=-f(x)\) 

\(\displaystyle f(-x)=|-x|+2\neq -f(x)\) so there is not symmetry about the x-axis

For a function to be symmetrical about the origin, you must replace y with (-y) and x with (-x) and the resulting function must be equal to the original function.

\(\displaystyle [(-y)=|(-x)|+2]\neq [y=|x|+2]\)

 

So there is no symmetry about the origin.

Example Question #1 : Determine The Symmetry Of An Equation

Algebraically check for symmetry with respect to the x-axis, y-axis, and the origin. \(\displaystyle y=\left | x \right |+2\)

Possible Answers:

Symmetry about the x-axis, y-axis, and origin

Symmetry about the x-axis and y-axis

Symmetry about the y-axis and the origin

Symmetry about the x-axis

Symmetry about the y-axis

Correct answer:

Symmetry about the y-axis

Explanation:

For a function to be symmetrical about the y-axis, it must satisfy \(\displaystyle f(x)=f(-x)\)

\(\displaystyle f(x)=\left |-x \right |+2=\left |x \right |+2\) so there is symmetry about the y-axis

For a function to be symmetrical about the x-axis, it must satisfy \(\displaystyle f(-x)=f(x)\)

\(\displaystyle f(-x)=\left |-x \right |+2\neq -f(x)\) so there is not symmetry about the x-axis

For a function to be symmetrical about the origin, you must replace y with (-y) and x with (-x) and the resulting function must be equal to the original function.

\(\displaystyle \left [ (-y)=\left |-x \right |+2 \right ]\neq \left [y=\left |x \right |+2 \right ]\)

So there is no symmetry about the origin, and the credited answer is "symmetry about the y-axis".

Example Question #1 : Determine The Symmetry Of An Equation

Which of the following best describes the symmetry of  \(\displaystyle x=-y^{2}+4\) with respect to the x-axis, y-axis, and the origin.

Possible Answers:

No symmetry

Symmetrical about the y-axis

Symmetrical about the x-axis

Symmetrical about the origin

Correct answer:

Symmetrical about the x-axis

Explanation:

For a function to be symmetrical about the y-axis, it must satisfy \(\displaystyle f(x)=f(-x)\)

\(\displaystyle f(-x)=\sqrt{4+x}=-f(x)\) so there is not symmetry about the y-axis

For a function to be symmetrical about the x-axis, it must satisfy \(\displaystyle f(-x)=f(x)\)

\(\displaystyle f(-x)=\sqrt{4+x}=-f(x)\) so there is symmetry about the x-axis

For a function to be symmetrical about the origin, you must replace y with (-y) and x with (-x), and the resulting function must be equal to the original function.

\(\displaystyle \left ( -x \right )=-(-y)^{2}+4\neq -y^{2}+4\)

So there is no symmetry about the origin, and the answer is Symmetrical about the x-axis.

Example Question #1 : Determine The Symmetry Of An Equation

Is the following function symmetrical about the y-axis (is it an even function)?

\(\displaystyle y=x^{3}+5x-7\)

Possible Answers:

No

The given information does not include a function

There is insufficient information to determine the answer

Yes

Correct answer:

No

Explanation:

For a function to be even, it must satisfy the equality \(\displaystyle f(x)=f(-x)\)

Likewise, if a function is even, it is symmetrical about the y-axis

\(\displaystyle f(-x)=(-x)^{3}+5(-x)-7=-x^{3}-5x-7\neq f(x)\)

Therefore, the function is not even, and so the answer is No.

Example Question #1 : Solving Quadratic Equations

Which of the following could be the equation for a function whose roots are at \(\displaystyle x= -4\) and \(\displaystyle x=3\)?

Possible Answers:

\(\displaystyle x^2-x-12=0\)

\(\displaystyle x^2+x-12=0\)

\(\displaystyle x^2+3x-4=0\)

\(\displaystyle 2x^2+2x-6=0\)

\(\displaystyle x^2-4x-3=0\)

Correct answer:

\(\displaystyle x^2+x-12=0\)

Explanation:

If the roots of the equation are at x= -4 and x=3, then we can work backwards to see what equation those roots were derived from. If we factored a quadratic equation and obtained the given solutions, it would mean the factored form looked something like:

\(\displaystyle (x+4)(x-3)=0\)

Because this is the form that would yield the solutions x= -4 and x=3. If we work backwards and multiply the factors back together, we get the following quadratic equation:

\(\displaystyle x^2+x-12=0\)

Example Question #1 : Write A Quadratic Equation When Given Its Solutions

Given roots \(\displaystyle x_1=1+i, x_2=1-i\). Write a quadratic polynomial that has\(\displaystyle x_1,x_2\) as roots.

Possible Answers:

\(\displaystyle x^2+3x+2\)

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

\(\displaystyle x^2+x+1\)

\(\displaystyle x^2+x+3\)

\(\displaystyle 2x^2+2x+1\)

Correct answer:

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

Explanation:

We can make a quadratic polynomial with \(\displaystyle x_1, x_2\) by mutiplying the linear polynomials they are roots of, and multiplying them out.

Start

\(\displaystyle (x-(1+i))(x-(1-i))\)  

Distribute the negative sign

\(\displaystyle (x-1-i)(x-1+i)\) 

FOIL the two polynomials. This means multiply the firsts, then the outers, followed by the inners and lastly, the last terms.

\(\displaystyle x^2-x+ix-x+1-i-ix+i-i^2\)    

\(\displaystyle x^2-x+ix-x+1-i-ix+i+1\)    

\(\displaystyle i^2 = -1\)

Simplify

\(\displaystyle x^2-2x+2\)    

 

Example Question #911 : Pre Calculus

If we know the solutions of a quadratic equation, we can then build that quadratic equation.

Find the quadratic equation when we know that:

\(\displaystyle x=4.5\) and \(\displaystyle x=2.3\) are solutions.

Possible Answers:

\(\displaystyle f(x)=x^2-6.8x+10.35\)

\(\displaystyle f(x)=x^2+6.8x+10.35\)

\(\displaystyle f(x)=x^2-6.8x-10.35\)

\(\displaystyle f(x)=x^2+6.8x-10.35\)

Correct answer:

\(\displaystyle f(x)=x^2-6.8x+10.35\)

Explanation:

Since we know the solutions of the equation, we know that:

\(\displaystyle (x-4.5)(x-2.3)=0\)

We simply carry out the multiplication on the left side of the equation to get the quadratic equation.

\(\displaystyle (x-4.5)(x-2.3)=x^2-4.5x-2.3x+10.35=x^2-6.8x+10.35\)

 

Example Question #1 : Write A Quadratic Equation When Given Its Solutions

Which of the following is a quadratic function passing through the points \(\displaystyle (0,0)\) and \(\displaystyle (6,0)\)?

Possible Answers:

\(\displaystyle f(x)=x^{2}+6x\)

\(\displaystyle f(x)=x-6\)

\(\displaystyle f(x)=x^{2}-6x\)

\(\displaystyle f(x)=x^{2}-6x+9\)

\(\displaystyle f(x)=x^{2}-6\)

Correct answer:

\(\displaystyle f(x)=x^{2}-6x\)

Explanation:

These two points tell us that the quadratic function has zeros at \(\displaystyle x=0\), and at \(\displaystyle x=6\).

These correspond to the linear expressions \(\displaystyle x\), and \(\displaystyle (x-6)\).

Expand their product and you arrive at the correct answer.

If the quadratic is opening up the coefficient infront of the squared term will be positive. Thus we get:

\(\displaystyle f(x)=x(x-6)=x^2-6x\).

If the quadratic is opening down it would pass through the same two points but have the equation:

\(\displaystyle f(x)=-x(x-6)=-x^2+6x\).

Since only \(\displaystyle f(x)=x^{2}-6x\) is seen in the answer choices, it is the correct answer.

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