Question

Evaluate cos2a if sin a=3/5, pi/2 11 years ago

Answer 1

http://tutorial.math.lamar.edu/pdf/Trig_Cheat_Sheet.pdf
see page 2 (double angle formulas)

\[\Large \cos(2a) = 1 - 2\sin^2(a)\]

\[\Large \cos(2a) = 1 - 2*\left(\sin(a)\right)^2\]

\[\Large \cos(2a) = 1 - 2*\left(\frac{3}{5}\right)^2\]

I'll let you finish up

Answer 2

Special note:

if pi/2<a<pi

then pi<2a<2pi

notice how I doubled everything

Answer 3

cos(2a)=7/25, right?

Answer 4

correct

Answer 5

Do I do anything to the cos(2a)? Or is that the answer?

Answer 6

@jim_thompson5910

Answer 7

nope, cos(2a) is left alone and yes that is the answer

Answer 8

Thank you!! Can you help me with another one?

Answer 9

sure