2x-tan(x-y)=∫(0,x-y) [sec（t）]^2dt两边对x求导得：2-sec²(x-y)(1-y')=sec²(x-y)(1-y')sec²(x-y)(1-y')=1y'=1-cos²(x-y)=sin²(x-y)y''=2sin(x-y)cos(x-y)(1-y')=sin(2x-2y)*cos²(x...