1、y'=(xcos2x)'=x'cos2x+x*cos2x'=cos2x-x*sin2x*2=cos2x-2xsin2x
2、limx(e^x-1)/(cosx-1)
=limx(1+x+x^2/x+x^3/6+...-1)/(1-x^2/2+x^4/24+...-1)
=limx^2/(-x^2/2)
=-2
3、dx+2ydy+2zdz=2xzdx+x^dz+2dz
dz=(dx+2ydy-2xzdx)/(x^+2-2z)
=(dx+2ydy-2xzdx)/[(x+y^)/z]
=(zdx+2yzdy-2xz^dx)/(x+y^)
4、y'=4x-1/x (x>0)
x>1/2时,y'>0 y单调递增
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