用字母a代替cosacosπ/6+sinasinπ/6+sinπ/6=4√3/5√3/2*cosa+3/2*sina=4√3/5两边除以√31/2*cosa+√3/2*sina=4/5sinπ/6*cosa+cos*sina=4/5sin(π/6+a)=4/5 所以sin(a+7π/6)=sin[π+(π/6+a]=-sin(a+π/6)=-4/5...