f(x,y)=ln(1+x^+y^)求df(1,2)

题目

f(x,y)=ln(1+x^+y^)求df(1,2)

答案

∂f/∂x=2x/(1+x^2+y^2)
∂f/∂y=2y/(1+x^2+y^2)
df(1,2)=∂f/∂x*dx+∂f/∂y*dy=1/3dx+2/3dy