a) x² - (2-x)² = 8
x² - (2² - 2 · 2 · x + x²) = 8
x² - (4 - 4x + x²) = 8
x² - 4 + 4x - x² = 8
-4 + 4x = 8
4x = 8 + 4
4x = 12 /:4
x = 3
b) (3x + 1)² - 9x² = 7
(3x)² + 2 · 3x · 1 + 1² - 9x² = 7
9x² + 6x + 1 - 9x² = 7
6x + 1 = 7
6x = 7 - 1
6x = 6 /:6
x = 1
c) (x + 1)² - (x + 1)(x - 1) = 12
x² + 2 · x · 1 + 1² - (x² - 1²) = 12
x² + 2x + 1 - x² + 1 = 12
2x + 2 = 12
2x = 12 - 2
2x = 10 /:2
x = 5
