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计算下列不定积分
(1) $\int \dfrac {x^{2}}{ \sqrt {1-x^{2}}}dx$.

(2) $\int \dfrac {x^{3}}{ \sqrt {1-x^{2}}}dx$.

(3) $\int \dfrac {dx}{1+ \sqrt {1-x^{2}}}$.

(4) $\int \dfrac {dx}{x \sqrt {1+x^{2}}}$.