Question #bf732

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Answer 1 · 2016-10-03T18:32:38

See below

#cos(x+y)+cos(x-y) = 2 cosxcosy#

Left Side #=cos(x+y)+cos(x-y)#

Use formulas #cos(A+B)=cosAcosB-sinAsinB#
and #cos(A-B)=cosAcosB+sinAsinB#

#=cosxcosy-sinxsiny + cosxcosy+sinxsiny#

#=cosxcosy+cosxcosy#

#=2cosxcosy#

#=# Right Side