(2x+7)^2 = 9(x+2)^2
4x^2 + 28x + 49 = 9(x^2 + 4x + 4)
4x^2 + 28x + 49 = 9x^2 + 36x + 36
4x^2 + 28x - 9x^2 - 36x = 36 - 49
-5x^2 - 8x = -13
-5x^2 - 8x + 13 = 0
-(5x^2 + 8x - 13) = 0
-(5x^2 - 5x + 13x - 13) = 0
-[(5x^2 - 5x) + (13x - 13)] = 0
-[5x(x-1) + 13(x-1)] = 0
-[(x-1)(5x+13)] = 0
-(x-1)(5x+13) = 0
-(x-1) = 0 hay -(5x+13) = 0
-x + 1 = 0 hay -5x -13 = 0
-x = -1 hay -5x = 13
x = 1 hay x = -13/5
Vậy S = { 1 ; -13/5}