Value of cos(-3888)?

1 Answer
Jul 18, 2018

# (sqrt5-1)/4#.

Explanation:

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

#(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#.