x+y=7,xy=-8,下列各式计算结果不正确的是:A.(x+y)^2=49
(x+y)^2=x^2+2xy+y^2=7^2+2*(-8)=49-16=33
x^2+y^2=(x+y)^2-2xy=49+16=65
(x-y)^2=x^2+y^2-2xy=(x+y)^2-4xy=49+32=81
由(x-y)^2=81得:
x-y=9 或x-y=-9
x=8,y=-1
或x=-2,y=9(因为xy=-8,不合题意舍去)
所以:x^2-y^2=(x+y)(x-y)=7*(8+1)=7*9=63
x+y=7,xy=-8,下列各式计算结果不正确的是:A.(x+y)^2=49
(x+y)^2=x^2+2xy+y^2=7^2+2*(-8)=49-16=33
x^2+y^2=(x+y)^2-2xy=49+16=65
(x-y)^2=x^2+y^2-2xy=(x+y)^2-4xy=49+32=81
由(x-y)^2=81得:
x-y=9 或x-y=-9
x=8,y=-1
或x=-2,y=9(因为xy=-8,不合题意舍去)
所以:x^2-y^2=(x+y)(x-y)=7*(8+1)=7*9=63