7. xy+lny = 1, x = 0 时， y = e
两边对 x 求导，y+xy'+y'/y = 0，
y' = -y^2/(1+xy), y'(0) = -e^2 ,
dy(x=0) = -e^2dx
8. e^y+xy = e
两边对 x 求导， y'e^y+y+xy' = 0，
y' = -y/(x+e^y)
dy = -[y/(x+e^y)]dx
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