cos(六分之派－阿法）＝二分之一,
cos（π/6-α）=1/2,
根据诱导公式,所以sin[π/2-（π/6-α）] =1/2,
即sin(π/3+α) =1/2,
cos（2π/3+2α）= cos[2(π/3+α)]
=1-2 sin²(π/3+α) =1/2.