三角不等式问题

原式=sinAsinB/sinC+sinBsin(A+B)/sinA+sinAsin(A+B)/sinB=sinAsinB/sinC+sinBsinBsin(A+B)/sinAsinB+sinAsinAsin(A+B)/sinBsinA (通分）=sinAsinB/sinC+sin(A+B)(sin²B+sin²A）/sinAsinB=sinAsinB/sinC+s...=sinAsinB/sinC+sin(A+B)(sin²B+sin²A）/sinAsinB =sinAsinB/sinC+sin(A+B)/sinAsinB这一步难道(sin²B+sin²A）=1额，不是，我想错了