设sina+cosa=t,-根号2《t《根号2
sina*cosa=(t^2-1)/2
f(x)=1/2[t^2+2t-1]=1/2[(t+1)^2-2]
显然当t=根号2时,f(x)取得最大值
f(x)max=.
当t=-1时,f(x)取得最小值,
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