Răspuns:
Explicație pas cu pas:
20%x+30%x•80%•x+420=x
20x/100+30×80x²/10000+420=x
(20x+24x²)/100-x=-420
20x+24x²-100x=-42000
24x²-80x+420 00=0 /4
6x²-20x-10500=0
3x²-10x-5250=0
x1,2=5±√25+15750/3=5±125,6/3
x1=130,6/3=43,53
x2=-40,2
