Question

已知x-y=5,xy=6,则(x+y)^2=?

Answer 1

把x-y=5这个式子平方 得到x方-2xy+y方=25又因为xy=-6 所以 x方+y方=13再把问题展开 x方+2xy+y方=13-12=1

Answer 2

x-y=5(x-y)^2=25x^2+y^2-2xy=25x^2+y^2=25+2xy所以(x+y)^2=x^2+y^2+2xy =25+4xy =1

Answer 3

（x+y）�0�5 =x �0�5+y �0�5+2xy=x �0�5+y �0�5-2xy+4xy=（x-y）�0�5+4xy=5�0�5-4×6=25-24=1

Answer 4

x-y=5(X-Y)^2=X^2+Y^2-2XY=25xy=-6(x+y)^2=X^2+Y^2+2XY=(X-Y)^2+4XY=25+4×（-6）=1