a)12х^2-3х+20х-5=12х^2-6х+42х-21
-19х=-16
19х=16
х=16/19
б)(5x-1)(2-x)=(x-3)(2-5x)
10х-5х^2-2+х=2х-5х^2-6+15х
10х-2+х-2х+6-15х=0
-6х=-4
Х=4/6=2/3
в)(5x+1)(2x-3)=(10x-3)(x+1)
10х²-15х+2х-3=10х²+10х-3х-3
10х²-10х²-15х+2х-10х+3х-3+3=0
-15х+2х-10х+3х=0
-25х+5х=0
-20х=0
х=0:(-20)
х=0
г)(7x-1)(x+5)=(3+7x)(x+3)
7x^2 + 35x - x - 5 = 3x + 9 + 7x^2 + 21Xx
7x^2 + 34x - 5 = 7x^2 + 24x + 9
34x - 24x = 9 + 5
10x = 14
x = 1.4