# Precalculus : Half-Angle Identities

## Example Questions

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### Example Question #1 : Double And Half Angle Identities

Compute

Explanation:

A useful trigonometric identity to remember for this problem is

or equivalently,

If we substitute  for , we get

### Example Question #251 : Pre Calculus

Compute

Explanation:

A useful trigonometric identity to remember is

If we plug in  into this equation, we get

We can divide the equation by 2 to get

### Example Question #9 : Double And Half Angle Identities

Using the half-angle identities, which of the following answers best resembles ?

Explanation:

Write the half angle identity for sine.

Since we are given , the angle is equal to .  Set these two angles equal to each other and solve for .

Substitute this value into the formula.

### Example Question #10 : Double And Half Angle Identities

Let  and  two reals. Given that:

What is the value of:

?

Explanation:

We have:

and :

(1)-(2) gives:

Knowing from the above formula that:( take a=b in the formula above)

This gives:

### Example Question #1 : Half Angle Identities

Let , , and  be real numbers. Given that:

What is the value of  in function of ?

Explanation:

We note first, using trigonometric identities that:

This gives:

Since,

We have :

### Example Question #251 : Pre Calculus

Using the fact that,

.

What is the result of the following sum:

Explanation:

We can write the above sum as :

From the given fact, we have :

and we have : .

This gives :

### Example Question #252 : Pre Calculus

Compute  in function of .

Explanation:

Using trigonometric identities we have :

and we know that:

This gives us :

Hence:

### Example Question #4 : Half Angle Identities

Given that :

Let,

What is  in function of ?

Explanation:

We will use the given formula :

We have in this case:

Since we know that :

This gives :

### Example Question #5 : Half Angle Identities

Using the fact that  , what is the result of the following sum:



Explanation:

We can write the above sum as :

From the given fact, we have :

This gives us :

Therefore we have:

### Example Question #1 : Half Angle Identities

Let be real numbers. If  and

What is the value of in function of   ?

Explanation:

Using trigonometric identities we know that :

This gives :

We also know that

This gives :

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