Đáp án:
Giải thích các bước giải:
VT =(cos2π/7+cos4π/7+cos6π/7) .(2sinπ/7)/(2sinπ/7)
=0,5/(sinπ/7)(2sinπ/7cos2π/7+2sinπ/7cos4π/7+2sinπ/7cos6π/7)
= 0,5/(sinπ/7)[(sin3π/7-sinπ/7)+(sin5π/7-sin3π/7)+(sin7π/7-sin5π/7)]
=0,5/(sinπ/7)×(-sinπ/7)
= -0,5(đpcm )
hơi khó hìn nha