a+2b=6,.1
a-3b=-3.2
1式-2式得
5b=9
b=9/5
将b=9/5代入1式得
a+2*9/5=6
a=12/5
5a^2-20b^2
=5(a^2-4b^2)
=5(a+2b)(a-2b)
=5*6*(12/5-2*9/5)
=5*6*(-6/5)
=-36
a+2b=6,a-2b=-3
5a^2-20b^2
=5(a^2-4b^2)
=5(a+2b)(a-2b)
=5*6*(-3)
=-90