设 $M=\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}} \frac{\sin x}{1+x^2} \cos ^4 x d x, N=\int_{\frac{-\pi}{2}}^{\frac{\pi}{2}}\left(\sin ^3 x+\cos ^4 x\right) d x, P=\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}}\left(x^2 \sin ^3 x-\cos ^4 x\right) d x$,则 M、N、P 的大小关系为
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