81c² - d² + 9c + d = (81c² - d²) + (9c + d) = (9c - d)(9c + d) + (9c + d) =
= (9c + d)(9c - d + 1)
((3х-1)/6)-(х/3)=((5-х)/9)|×18
3(3х-1)-6х=2(5-х)
9х-3-6х=10-2х
9х-6х+2х=10+3
5х=13|÷5
х=(13/5)
Проверка:
((3×(13/5)-1)/6)-((13/5)/3)=((5-(13/5))/9)
(((3×13-1×5)/5)/6)-(13/15)=(((5×5-13)/5)/9)
(((39-5)/5)/6)-(13/15)=(((25-13)/5)/9)
(34/30)-(13/15)=(12/45)
((34-13×2)/30)=(12/45)
(8/30)=(12/45)
(4/15)=(4/15)- истина.
Ответ: х=(13/5)
Нужно
x(x+7)=0
x=0 или x+7=0
x=-7
То есть при x=0 and x=-7