a^2+b^2=a^2+b^2+2ab-2ab
=(a+b)^2-2ab
将a+b=7,ab=2代入原式,得:
原式=49-2*2
=45
a^2-ab+b^2=a^2-ab+b^2+3ab-3ab
=(a+b)^2-3ab
将a+b=7,ab=2代入原式,得:
原式 =49-6
=43
原式=(100-1)^2+200-1
=99800+200-1
=10000
原式=20x-4x^2-25-(4x^2-20x+25)
=20x-4x^2-25-4x^2+20x-25
=40x-8x^2-50