cos(xy)=x+y
两边分别对x求导：
-sin(xy)*(y+xy′)=1+y′
y′=-[1+ysin(xy)]/[xsin(xy)+1]
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左边对cos求导：-sin(xy)
再对xy求导：y+xy′
右边对x求导：1+y′
左边两项相乘,最后解出y′