Question

化简（或求值）

(1).7xy-3x^2+2y^2-4xy +4x^2-3y^2
(2).(5a^2-2ab)-2(3a2+4ab-二分之一b^2)
(3).2x-(3x-二分之x-2)+[5x-二分之3(x-2)]
(4).8ab-4a^2b^2-8ab^2+2ab-2ab^2+4a^2b^2，（其中ab-ab^2=-2)

Answer 1

(1).7xy-3x^2+2y^2-4xy +4x^2-3y^2
合并同类项
=3xy+x^2-y^2

(2).(5a^2-2ab)-2(3a2+4ab-二分之一b^2)
=5a^2-2ab-6a^2-8ab+b^2
=-a^2-10ab+b^2

(3).2x-(3x-二分之x-2)+[5x-二分之3(x-2)]
=2x-5/2x+2+5x-3/2x-3
=-2x-1

(4).8ab-4a^2b^2-8ab^2+2ab-2ab^2+4a^2b^2
=10ab-10ab^2
=10(ab-ab^2)
=-20

Answer 2

3(2-x)=2(3+x)
6-3x=6+2x
2x+3x=6-6
5x=0
x=0÷5
x=0
b²=a²+c²-2accosB
7=4+c²-2c
c²-2c-3=(c-3)(c+1)=0
c>0
c=3

S=1/2accosB
=1/2*2*3*√3/2
=3√3/2