Tuesday

September 30, 2014

September 30, 2014

Posted by **lan** on Sunday, May 8, 2011 at 9:56am.

- trig -
**Reiny**, Sunday, May 8, 2011 at 10:23amstart with

cos 2A = 2cos^2 A - 1

cos150° = 2 cos^2 75 - 1

-√3/2 + 1 = 2cos^2 75°

cos^2 75° = (1 - √3/2)/2

cos 75° = √[(1 - √3/2)/2]

An easier ways would have been

cos75 = cos(45+30)

= cos45cos30 - sin45sin30

= (1/√2)(√3/2) - (1/√2)(1/2)

= (√3 - 1)/(2√2)

Use your calculator to show that both produce cos75

**Answer this Question**

**Related Questions**

trig - Use a half-angle formula to find the exact value of cos 5ð/12

Trig - If cosx = 10/19 an pi < x < 2pi, find the exact value of cos x/2 ...

Trig - Use the half angle formula to find the exact value of tan 7 pie/8

MATH - 1.)Find the exact solution algebriacally, if possible: (PLEASE SHOW ALL ...

Mathmatics - a. Use the formulas for lowering powers to rewrite the expression ...

Please help Math - a. Use the formulas for lowering powers to rewrite the ...

trig - Find the exact value by using a half-angle identity. cos 22.5 degrees

Trigonometry - use the appropriate trig identity (sum and difference, half angle...

Trig - How do i find the exact value of sin 15 using the half angle formula

Trig - Use the half-angle formulas to come up with an exact expression for each ...