2x = 7 + x
2x - x = 7
x = 7

b) (x - 3)/5 + (1 + 2x)/3 = 6
3(x - 3)/15 + 5(1 + 2x)/15 = 90
3(x - 3) + 5(1 + 2x) = 90
3x - 9 + 5 + 10x = 90
13x - 4 = 90
13x = 90 + 4
13x = 94
x = 94/13

a) 3x (x - 1) - 1 + x = 0
    3x (x - 1) - (x - 1) = 0
    (x - 1)(3x - 1) = 0
=> x - 1 = 0 hoặc 3x - 1 = 0
=> x = 1      hoặc x = 1/3

b) x² - 9x = 0
    x(x - 9) = 0
=> x = 0 hoặc x - 9 = 0
=> x =0 hoặc x = 9

a) x² + 25 - 10x = x² - 10x + 25 = (x)² - 2 . x . 5 + 5² = (x - 5)²

b) -8y³ + x³ = (-2y)³ + (x)³  = (-2y + x) (4y² - (-2xy) + x²)
                                            = (-2y + x) (4y² + 2xy + x²)

a) (2x + 1)² = (2x)² +  2 . 2x . 1 + 1² = 4x² + 4x + 1

b) (a - b/2)² = (a)² - 2 . a . b/2 + (b)² = a² - ab + b²