将式子乘以sin(1/2)再除以sin(1/2)cos1*sin(1/2)=1/2(sin(3/2)-sin(1/2))cos2*sin(1/2)=1/2(sin(5/2)-sin(3/2))cos3*sin(1/2)=1/2(sin(7/2)-sin(5/2))所以原式化为(sin((2n+1)/2)-sin(1/2))/2/sin(1/2)=sin((2n+1)/2...