Question

已知X-Y=-3,则X^2+Y^2/2-XY

(7a+A)^2=49a^2-14ab^2+B,A=?,B=?这个

Answer 1

X-Y=-3
则(X-Y)²=9
则X²-2XY+Y²=9
则X²+Y²=9+2XY
X^2+Y^2/2-XY
=(9+2XY)/2-XY
=9/2+XY-XY
=9/2

追问

下面的

追答

(7a+A)^2=49a²+14aA+A²=49a^2-14ab^2+B
所以A=-b²，A²=B
所以A=-b²,B=b^4

Answer 2

(X^2+Y^2)/2-XY
=(X^2-2XY+Y^2)/2
=(X-Y)^2/2
=9/2