1. ## Compound angle formulae

1.cos 19pi/12

2.If sin A =3/5 with 0<A<pi/2 and cos B=1/3 with 0<B<pi/2 calculate...
a)cos(A+B) b)sin(A+B) c)cos2B d)sin2A

Can someone help me with 1., 2a) and d)... I'll try b) and c) and post it up later to see if it's right.

Thank you!

sinA = 3/5 means that cosA = 4/5
cosB = 1/3 means that sinB = $\frac{\sqrt{8}}{3}$

using cos(A+B) = cosAcosB - sinAsinB, I got $cos(A+B) = \frac{4-3\sqrt{8}}{15}$

It's a similar story with the others. Just use those values of sinA, sinB, cosA, cosB and keep substituting in the relevant double angle formulae.

3. Originally Posted by johett
1.cos 19pi/12
[snip]
cos 19pi/12 = cos 5pi/12. Note that 5pi/12 = pi/6 + pi/4. Apply cos(A+B) = cosAcosB - sinAsinB.