Posted by **Anonymous** on Sunday, October 17, 2010 at 2:05am.

how to do the calculation while taking 'A'

as 15 in the half angle formula used for calculating the value of sin 7.5 ?

- trig -
**Reiny**, Sunday, October 17, 2010 at 8:41am
Unless you have found sin15 or cos15 before, and it has become part of your trig repertoire , you will have to do it twice

since you must know that cos 30 = √3/2

and using

cos 2A = 2cos^2 A - 1 , let 2A = 30

cos30 = 2cos^2 15 - 1

solving this for cos 15 I got √(√3 + 2)/2

now use the other expansion for cos 2A

cos 2A = 1 - 2sin^2 A

cos 15 = 1 - 2sin^2 7.5

this gave me

2sin^2 7.5 = 1 - √(√3 + 2)/2

sin 7.5 = √[2 - √(√3 + 2) ]/2

you can check it with sin 7.5 on your calculator, it is correct.

## Answer This Question

## Related Questions

- Trig - How do i find the exact value of sin 15 using the half angle formula
- Trigonometry - Identities - If tan 2x = - 24/7, where 90 degrees < x < 180...
- trig - find the exact value by using the appropriate half-angle identity: sin(...
- trig - Solving Trig Functions: How Do You Solve For Sin 7.5 Using Half-Angle and...
- trig - Use a half-angle formula to find the exact value of cos 5ð/12
- trig - half angle formula to find exact value cos 112.5 degree
- trig - use a half angle formula to find the exact value of cos 75 degree
- Trig - Use the half angle formula to find the exact value of tan 7 pie/8
- trig - I asked below but I guess it was not understood what I was asking. I'll ...
- Math - Use a half angle formula to determine the exact value of sin 7(pi)/12.

More Related Questions