Question #0f6fb

1 Answer
Nov 21, 2017

#1/2{cos3x-cos7x}.#

Explanation:

Using the Identity :

#-2sinalphasinbeta=cos(alpha+beta)-cos(alpha-beta), i.e., #

#sinalphasinbeta=1/2{cos(alpha-beta)-cos(alpha+beta)},# we have,

#sin2xsin5x=1/2{cos(2x-5x)-cos(2x+5x)},#

#=1/2{cos(-3x)-cos7x}.#

But, #cos(-theta)=costheta.#

#:. sin2xsin5x=1/2{cos3x-cos7x}.#