先化简再求值:①5(3a^2b-ab^2)-4(-ab^2+3a^2b),其中a=-2,b=3 ②3y^2-x^2+(2x-y
问题描述:
先化简再求值:①5(3a^2b-ab^2)-4(-ab^2+3a^2b),其中a=-2,b=3 ②3y^2-x^2+(2x-y
)-(x^2+3y^2),其中x=1,y=-2
答
(1)
5(3a^2b-ab^2)-4(-ab^2+3a^2b)
=15a^2b-5ab^2+4ab^2-12a^2b
=15a^2b-12a^2b-5ab^2+4ab^2
=3a^2b-ab^2
=ab(3a-b)
=-2*3*[3*(-2)-3]
=-6*(-9)
=54
(2)
3y^2-x^2+(2x-y)-(x^2+3y^2)
=3y^2-x^2+2x-y-x^2-3y^2
=3y^2-3y^2-x^2-x^2+2x-y
=-2x^2+2x-y
=-2*1+2*1+2
=6