Value of cos(-3888)?

1 Answer
Jul 18, 2018

(sqrt5-1)/4514.

Explanation:

We have, (1):cos(-theta)=costheta,(1):cos(θ)=cosθ,

(2): cos(360^@k+theta)=costheta, k in ZZ,,

(3):cos(270^@+theta)=sintheta.

Therefore, cos(-3888^@)=cos(3888^@),

=cos((3600^@+(270^@+18^@)),

=cos(270^@+18^@),

=sin18^@.

rArr cos(-3888^@)=(sqrt5-1)/4.