Question

（1）1.2345^2+0.7655^2+2.469*0.7655

Answer 1

(1)

1.2345^2+0.7655^2+2.469*0.7655

=1.2345^2+0.7655^2+2*1.2345*0.7655

=(1.2345+0.7655)^2

=2^2

=4

(2)

X+ 1/X=3

(X+ 1/X)^2=X^2+ 1/X^2+2=9

因此X^2+ 1/X^2=7

(3)

X-Y=4

(X-Y)^2=X^2+Y^2-2XY=16

以为X^2+Y^2=12

所以XY=-2