令x=sina
由题意得a属于[0,π](因为cosa必须>0)
则1-x^2=1-sin²a=cos²a
∴y=cosa+sina=√2sin(a+π/4)
则a+π/4属于[π/4,5π/4]
-√2<=sin(a+π/4)<=1
-1<=√2sin(a+π/4)<=√2
∴值域[-1,√2]
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