3个平方差的计算题,

(a+b+c)(a-b-c)
=[a+(b+c)][a-(b+c)]
=a^2-(b+c)^2
(a-2b+3)(-a-2b+3)
=-(a-2b+3)(a+2b-3)
=-[(a-2b)(a+2b)-9]
=-(a^2-4b^2-9)
=-a^2+4b^2+9
x^2 -y^2=24
x^2 -y^2=(x+y)(x-y)=24
x+y=6
x-y=4
x=5,y=1
x+2y=7