f（x）=√（1-x^2）
1-x^2≥0
-1≤x≤1
√(1-x^2)与1-x^2有相同的单调性
1-x^2开口向下,极点为(0,1)
［-1,0］,f（x）=1-x^2单调递增,f（x）=√（1-x^2）单调递增
［0,1］,f（x）=1-x^2单调递减,f（x）=√（1-x^2）单调递减