Question

x^2+y^2=9,xy=4 x+y/(x+y)(x^2-xy+y^2)+(x+y)怎么化简？

图中第15题

Answer 1

(x+y)/(x³+y³+x+y)
=(x+y)/[(x+y)(x²-xy+y²)+(x+y)]
=1/(x²-xy+y²+1)
=1/[(x²+y²)-xy+1]
=1/(9-4+1)
=1/6
选C

Answer 2

see

追答

提取公因式

Answer 3

(x+y)²=x²+y²+2xy=9+4*2=17
x+y=±√17