Do you know how to do this problem?

cos #((13pi)/"12")#

1 Answer
Apr 23, 2018

Formula:- #"(1) "cos(pi+theta)=-costheta#
#" (2) "cos2theta=2cos^2theta-1#

Problem:-#darr#

#cos((13pi)/12)#

#=cos(pi+pi/12)#

#=-cos(pi/12)#

#=-sqrt((1+cos(pi/6))/2)#

#=-sqrt((1+sqrt3/2)/2#

#=-sqrt(1/2+sqrt3/4#

#=- 1/4sqrt(8+4sqrt3)#

#=- 1/4sqrt((sqrt6+sqrt2)^2#

#=- 1/4(sqrt6+sqrt2)#

Hope this helps...
Thank you...

:-)