Question

cos (п/6 + A) + cos(п/6-A) = корень 3 cosA

Answer 1

сosa+cosb=2cos[(a+b)/2]*cos[(a-b)/2]
------------------------------------------------------
2cos[π/6+a+π/6-a)/2]*cos[(π/6+a-π/6+a)/2]=2cosπ/6*cosa=
=2*√3/2*cosa=√3cosa
√3cosa=√3cosa