Question

If sinα=3/5 then find cos 3α

Answer 1

Given that

sinα = 3/5

Then cosα = √(1-sin²α) = √[1-(9/25)] = √(16/25) = 4/5       ...(1)

We know that

cos3α = 4cos³α - 3cosα

So cos3α = 4(4/5)³-3(4/5)        [ using (1) ]

cos3α = (256/125)-(12/5)

cos3α = (256-300)/125

cos3α =  -44/125