t=cosxy=a+bt,b>0,y在R上单调递增-1<=t<=1t=1,ymax=a+b=3/2t=-1,ymin=a-b=-1/2a=1/2,b=1
显然函数y=a+bcosx (b >0 )最大值 3/2=a+b最小值 -1/2=a-b两式相加的 a=1/2 b=1
