Verified answer

7) 49a² - 4b² = (7a)² - (2b)² = (7a - 2b)(7a + 2b)

8) ( a³ - b⁴)(a³ + b⁴) = (a³)² - (b⁴)² = a⁶ - b⁸

9) x² - 49 = 0

(x - 7)(x + 7) = 0

или     x - 7 = 0   и тогда    x = 7

или     x + 7 = 0   и тогда   x = - 7

Ответ : 7 ; - 7

10) 25y² - 4 = 0

(5y)² - 2² = 0

(5y - 2)(5y + 2) = 0

5y - 2 = 0     5y = 2     y₁ = 2/5

5y + 2 = 0    5y = - 2   y₂ = - 2/5

Ответ : 2/5 ; - 2/5

11) 87 * 93 = (90 - 3)(90 + 3) = 90² - 3² = 8 100 - 9 = 8 091

12) (x - 3)² - 9 = x² - 6x + 9 - 9 = x² - 6x