因为π/20,根据cos(a-b)=-4/5得sin(a-b)=3/5因为3π/2cos2a = cos((a+b) + (a-b))= cos(a+b)cos(a-b) - sin(a+b)sin(a-b)= 4/5 * (-4/5) - 3/5 * (-3/5)= -7/25
COS2A=2SINASINB