已知cos(a+π/3)=1/3 a是锐角 求cosa
问题描述：
已知cos(a+π/3)=1/3 a是锐角 求cosa
答
cosa
=cos[(a+π/3)-π/3]
=cos(a+π/3)cosπ/3+sin(a+π/3)sinπ/3
=(1/3)*(1/2)+(2√2/3)*(√3/2)
=(1+2√6)/6.