Question

Verify cos3A = 4cos³A – 3cosA, when A = 30°

Answer 1

L.H.S = cos3A

= cos3(30°)

= cos90°

= 0

R.H.S = 4cos³A – 3cosA

= 4cos³30° – 3cos30°

=`4(sqrt(3)/2)^3 - 3(sqrt(3)/2)`

= `(4 xx 3sqrt(3))/8 - (3sqrt(3))/2`

= `(3sqrt(3))/2 - (3sqrt(3))/2`

= 0

∴ L.H.S = R.H.S

Hence it is proved.