已知δ=x•ln(x+y)，则∂2δ/∂x2()

已知δ=x•ln(x+y)，则∂2δ/∂x2()
A、x/(x+y)2
B、-[x/(x+y)]2
C、(x+2y)/(x+y)2
D、(2x+y)/(x+y)2
【正确答案】：C
【题目解析】：二阶偏导数．因为∂δ/∂x=ln(x+y)+x/(x+y)，∂2δ/∂x2=1/(x+y)+y/(x+y)2=(x+2y)/(x+y)2.所以，选项C是正确的