Question

if cosine A=0•8 find sine B

Answer 1

I guess you mean find sin A because I have no idea what B is

cos^2 + sin^2 = 1
0.8^2 + sin^2 = 1
sin^2 = 1-0.64 = 0.36
sin = +/- sqrt(0.36)

Answer 2

as shown above, sinA = ±0.6

or, you can see that if you draw a triangle, it is just a scaled-down 3-4-5 triangle. so if sinA = 4/5, cosA = 3/5.