Question

вычислите cos 6pi/5 * cos 7pi/10 + sin 6pi/5 * sin 7pi/10

Answer 1

cos 6π/5 * cos 7π/10 + sin 6π/5 * sin 7π/10=cos(12π/10-7π/10)/2=

cos(5π/20)=cos(π/4)=√2/2