若x+y=3,则x²-y²+6y=
问题描述：
若x+y=3,则x²-y²+6y=
急
答
答：
x+y=3
x²-y²+6y
=(x-y)(x+y)+6y
=3(x-y)+6y
=3x-3y+6y
=3x+3y
=3(x+y)
=3*3
=9