Precalculus : Prove Trigonometric Identities

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #1 : Proving Trig Identities

Simplify:  \displaystyle 1+\frac{cos^2(\theta)}{sin^2(\theta)}

Possible Answers:

\displaystyle \frac{1}{cot^2{\theta}}

\displaystyle csc^2(\theta)

\displaystyle sec^2(\theta)

\displaystyle \pi+tan^2(\theta)

\displaystyle sin^2(\theta)

Correct answer:

\displaystyle csc^2(\theta)

Explanation:

To simplify \displaystyle 1+\frac{cos^2(\theta)}{sin^2(\theta)}, find the common denominator and multiply the numerator accordingly.

\displaystyle 1+\frac{cos^2(\theta)}{sin^2(\theta)}=\frac{sin^2(\theta)}{sin^2(\theta)}+\frac{cos^2(\theta)}{sin^2(\theta)}=\frac{sin^2(\theta)+cos^2(\theta)}{sin^2(\theta)}

The numerator is an identity.

\displaystyle sin^2(\theta)+cos^2(\theta)=1

Substitute the identity and simplify.

\displaystyle \frac{sin^2(\theta)+cos^2(\theta)}{sin^2(\theta)}= \frac{1}{sin^2(\theta)}= csc^2(\theta)

 

Example Question #2 : Proving Trig Identities

Evaluate in terms of sines and cosines:  

\displaystyle \frac{sec^2(\theta)}{cot(\theta)}

Possible Answers:

\displaystyle \frac{sin(\theta)}{cos(\theta)}

\displaystyle \frac{sin(\theta)}{cos^3(\theta)}

\displaystyle \frac{1}{cos(\theta)sin(\theta)}

\displaystyle \frac{cos(\theta)}{sin(\theta)}

\displaystyle \frac{sin(\theta)}{cos^2(\theta)}

Correct answer:

\displaystyle \frac{sin(\theta)}{cos^3(\theta)}

Explanation:

Convert \displaystyle \frac{sec^2(\theta)}{cot(\theta)} into its sines and cosines.

\displaystyle \frac{sec^2(\theta)}{cot(\theta)}= \left(\frac{1}{cos(\theta)}\times\frac{1}{cos(\theta)}\right)\div \frac{cos(\theta)}{sin(\theta)}

\displaystyle \left(\frac{1}{cos(\theta)}\times\frac{1}{cos(\theta)}\right)\div \frac{cos(\theta)}{sin(\theta)}=\frac{1}{cos^2(\theta)}\times\frac{sin(\theta)}{cos(\theta)}= \frac{sin(\theta)}{cos^3(\theta)}

Example Question #3 : Proving Trig Identities

Simplify the following:

\displaystyle sin^3(x)+sin(x)\cdot cos^2(x)

Possible Answers:

\displaystyle sin(x)

The expression is already in simplified form

\displaystyle 1

\displaystyle cos(x)

Correct answer:

\displaystyle sin(x)

Explanation:

\displaystyle sin^3(x)+sin(x)\cdot cos^2(x)

First factor out sine x.

\displaystyle =(sin^2(x)+cos^2(x))\cdot sin(x)

Notice that a Pythagorean Identity is present.

The identity needed for this problem is: 

\displaystyle (sin^2(x)+cos^2(x))=1

Using this identity the equation becomes,

\displaystyle =(1)\cdot sin(x)

\displaystyle =sin(x).

 

Example Question #4 : Proving Trig Identities

Simplify the expression \displaystyle \frac{sin(x)}{cos(x)}\times \frac{1}{cot(x)}

 

Possible Answers:

\displaystyle tan^2(x)

\displaystyle csc^2(x)

\displaystyle cos^2(x)

\displaystyle sin^2(x)

Correct answer:

\displaystyle tan^2(x)

Explanation:

To simplify, use the trigonometric identities \displaystyle \frac{sin(x)}{cos(x)}=tan(x) and \displaystyle \frac{1}{cot(x)}=tan(x) to rewrite both halves of the expression:

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

Then combine using an exponent to simplify:

\displaystyle tan^2(x)

Example Question #1 : Prove Trigonometric Identities

Simplify \displaystyle \frac{cos(x)}{sin(x)}

Possible Answers:

\displaystyle csc(x)

\displaystyle sec(x)

\displaystyle cot(x)

\displaystyle tan(x)

Correct answer:

\displaystyle cot(x)

Explanation:

This expression is a trigonometric identity: \displaystyle \frac{cos(x)}{sin(x)}=cot(x)

Example Question #32 : Trigonometric Functions

Simplify \displaystyle \frac{2}{csc^2(x)}+\frac{2}{sec^2(x)}

Possible Answers:

\displaystyle 2csc(x)

\displaystyle 2

\displaystyle csc(x)

\displaystyle 1

Correct answer:

\displaystyle 2

Explanation:

Factor out 2 from the expression:

\displaystyle 2(\frac{1}{csc^2(x)}+\frac{1}{sec^2(x)})

Then use the trigonometric identities \displaystyle \frac{1}{csc(x)}=sin(x) and \displaystyle \frac{1}{sec(x)}=cos(x) to rewrite the fractions:

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

Finally, use the trigonometric identity \displaystyle sin^2(x)+cos^2(x)=1 to simplify:

\displaystyle 2(1) = 2

Example Question #33 : Trigonometric Functions

Simplify \displaystyle \frac{2sin^2(x)}{tan(x)}+\frac{2cos^2(x)}{tan(x)}

Possible Answers:

\displaystyle 2tan(x)

\displaystyle 2cot(x)

\displaystyle 4cot(x)

\displaystyle 4tan(x)

Correct answer:

\displaystyle 2cot(x)

Explanation:

Factor out the common  \displaystyle \frac{2}{tan(x)} from the expression:

\displaystyle (\frac{2}{tan(x)})(sin^2(x)+cos^2(x))

Next, use the trigonometric identify \displaystyle sin^2(x)+cos^2(x)=1 to simplify:

\displaystyle \frac{2}{tan(x)}(1)

Then use the identify \displaystyle \frac{1}{tan(x)}=cot(x) to simplify further:

\displaystyle 2cot(x)

Example Question #2 : Prove Trigonometric Identities

Simplify \displaystyle \frac{2tan^2x+1}{tan^2x}+tan^2x

Possible Answers:

\displaystyle cscx

\displaystyle sec^2x+csc^2x

\displaystyle 2sec^2x

\displaystyle 2csc^2x

Correct answer:

\displaystyle sec^2x+csc^2x

Explanation:

To simplify the expression, separate the fraction into two parts:

\displaystyle \frac{2tan^2x}{tan^2x}+\frac{1}{tan^2x}+tan^2x

The \displaystyle tan^2x terms in the first fraction cancel leaving you with:

\displaystyle 2+\frac{1}{tan^2x}+tan^2x

Then you can deal with the remaining fraction using the rule that \displaystyle \frac{1}{tan^2x}=cot^2x. This leaves:

\displaystyle 2+cot^2x+tan^2x

You can separate this into:

\displaystyle 1+tan^2x+1+cot^2x

And each half of this expression is now a trigonometric identity: \displaystyle 1+tan^2x = sec^2x and \displaystyle 1+cot^2x=csc^2x. This gives you:

\displaystyle sec^2x+csc^2x

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