High School Math : Using Pythagorean Identities

Study concepts, example questions & explanations for High School Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Using Pythagorean Identities

Simplify

\(\displaystyle \cot(x)\times{\sin(x) }\)

Possible Answers:

\(\displaystyle \arcsin(\pi )\)

\(\displaystyle \arcsin(x)\)

\(\displaystyle \cos(x)\)

\(\displaystyle \csc(x)\)

\(\displaystyle \tan(x)\)

Correct answer:

\(\displaystyle \cos(x)\)

Explanation:

\(\displaystyle \cot(x) = \cos(x)/\sin(x)\).  Thus: \(\displaystyle [\cos(x)/\sin(x)]\times\sin(x)=\cos(x)\)

Example Question #2 : Using Pythagorean Identities

Simplify

\(\displaystyle \frac{cot (x)\cdot tan(x)}{cos(x)}\)

Possible Answers:

\(\displaystyle \cos(x)\)

\(\displaystyle \sec(x)\)

\(\displaystyle \cot(x)/sec(x)\)

\(\displaystyle \csc(x)\)

\(\displaystyle \cos(x)\times\tan(x)\)

Correct answer:

\(\displaystyle \sec(x)\)

Explanation:

\(\displaystyle \cot(x)\times\tan(x) = 1\) 

and

 \(\displaystyle 1/(\cos(x)) = \sec(x)\).

Example Question #6 : Trigonometric Identities

Simplify \(\displaystyle \frac{\cos^2x-1}{\sin x}\).

Possible Answers:

\(\displaystyle -\sin x\)

\(\displaystyle \sin^2 x\)

\(\displaystyle \sin x\)

\(\displaystyle \sin x\cos x\)

\(\displaystyle \tan x\)

Correct answer:

\(\displaystyle -\sin x\)

Explanation:

Remember that \(\displaystyle \sin^2x+\cos^2 x=1\). We can rearrange this to simplify our given equation:

\(\displaystyle \frac{\cos^2x-1}{\sin x}\)

\(\displaystyle =\frac{-\sin^2x}{\sin x}=-\sin x\)

Example Question #1 : Trigonometric Identities

Simplify:

\(\displaystyle \sin^2x+\cos^2x+\tan^2x\)

Possible Answers:

\(\displaystyle \csc^2 x\)

\(\displaystyle \sin x\cos x\)

\(\displaystyle \sec^2x\)

\(\displaystyle 2\cos x+\cot x\)

This is the most simplified version.

Correct answer:

\(\displaystyle \sec^2x\)

Explanation:

Whenever you see a trigonometric function squared, start looking for a Pythagorean identity.

The two identities used in this problem are \(\displaystyle \sin^2 x + \cos ^2 x=1\) and \(\displaystyle \tan^2 x +1 =\sec^2 x\).

Substitute and solve.

\(\displaystyle \sin^2x+\cos^2x+\tan^2x=?\)

\(\displaystyle (\sin^2x+\cos^2x)+\tan^2x=?\)

\(\displaystyle (1)+\tan^2x=\sec^2 x\)

Learning Tools by Varsity Tutors