设f(x，y)=x+y-√x2+y2，求fx(3，4)．

设f(x，y)=x+y-√x²+y²，求f_x(3，4)．
【正确答案】：f_x(x,y)=1-1/2√x²+y²(x²+y²)_x′=1 -x/√(x²+y^{2x(3，4)=1-3/(3²+4²)=1-(3/5)=2/5}