(1)x²-81=0
x²=81,x=±√81=±9
(2)25x²=36
x²=25/36,
x=±√25/36=±5/6
(3)x³ +64=0
x³ =-64,
x=3次√-64=-4
(4)（x-1）³ =8
x-1 =3次√8=2
x=2+1=3