Question

what is the value of cos 15​

Answer 1

using formula of cos(a-b)=cos a cos b + sin a sin b.

Here we can write 15=60–45

So let a=60 & b=45

Cos(60–45)= cos 60 cos 45 + sin 60 sin 45

Cos(15)= 1/√2 x 1/2 + 1√2 x √3/2

Cos 15= (1+√3)/2√2

Answer 2

using formula cos(a-b)=cos a cos b + sin a sin b

Here we can write 15=60–45

So let a=60 & b=45

Cos(60–45)= cos 60 cos 45 + sin 60 sin 45

Cos(15)= 1/√2 x 1/2 + 1√2 x √3/2

Cos 15= (1+√3)/2√2