y'=sec^2(x+y)*(1+y')
y'(1-sec^2(x+y))=sec^2(x+y)
y'*tan(x+y)=-sec^2(x+y)
y'=-1/cos^2(x+y)*1/tan(x+y)
=-1/[sin(x+y)cos(x+y)]
=-2/sin(2x+2y)
=-2csc(2x+2y)
y''=2cot(2x+2y)csc(2x+2y)*(2+2y')
把y'代入即可
y' = [1/cos²(x+y)]*(x+y]' = [1/cos²(x+y)]*(1 + y') =
[cos²(x+y)]y' = 1 + y'
[cos²(x+y) -1]y' = 1
y' = 1/[cos²(x+y) -1]
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