1.已知:a+b=12,a的平方+b的平方=74
求ab的值
[a+b]^2=a^2+b^2+2ab
12^2=74+2ab
ab=35
2.(x+y-2)(x-y+2)
=[x+(y-2)][x-(y-2)]
=x^2-(y-2)^2
=x^2-(y^2-4y+4)
=x^2-y^2+4y-4
3.先化简,再求值
(4x-3y)的平方-(3x-2y)(3x+2y),其中x=2,y=1
=16x^2-24xy+9y^2-[9x^2-4y^2]
=7x^2-24xy+13y^2
=7*2^2-24*2*1+13*1^2
=28-48+13
=-7