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!

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- Jan 13th 2008, 12:04 PMjohettCompound 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! - Jan 13th 2008, 01:29 PMWWTL@WHL
Drawing a little diagram might help you.

sinA = 3/5 means that cosA = 4/5

cosB = 1/3 means that sinB = $\displaystyle \frac{\sqrt{8}}{3} $

using cos(A+B) = cosAcosB - sinAsinB, I got $\displaystyle 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. - Jan 13th 2008, 04:02 PMmr fantastic