Posted by **Anonymous** on Thursday, May 3, 2007 at 1:43pm.

cosA= 5/9 find cos1/2A

are you familiar with the half-angle formulas?

the one I would use here is

cos A = 2cos^2 (1/2)A - 1

5/9 + 1 =2cos^2 (1/2)A

14/9 =2cos^2 (1/2)A

cos (1/2)A = √(7)/3

## Answer this Question

## Related Questions

- Math - Solving Trig Equations - What am I doing wrong? Equation: sin2x = 2cos2x ...
- Trigonometry - Need help solving problems using the Trigonometric Identities: ...
- Math, derivatives - f(x) = x² + 2Cos²x, find f ' (x) a) 2(x+cos x) b) x - sin x ...
- Math - Solving for Trig Equations - Solve the following equation for 0 less than...
- Math - Trig - Double Angles - Prove: cos4x = 8cos^4x - 8cos^2x + 1 My Attempt: ...
- Trig - If cosx = 10/19 an pi < x < 2pi, find the exact value of cos x/2 ...
- Pre-Cal - verify the identity sin x(1 - 2 cos^2 x + cos^4 x) = sin^5 x 1-2cos^2 ...
- trig - I dont understand this. can you please tell how you got each answer? ...
- math - how would you find the angle to (3)^.5sin(2x)-2cos(2x)+2sin^2(x)=1 I know...
- Calculus - which of the following integrals results from making the substitution...