√ ( 7x + 8 ) = x
( √ ( 7x + 8 ))² = x²
7x + 8 ≥ 0 ; 7x ≥ - 8 ; x ≥ - 1 1/7 ; ( - 1 1/7 ; + ∞ ) ; x > 0
7x + 8 = x²
x² - 7x - 8 = 0
D = 49 + 32 = 81 = 9²
x1 = ( 7 + 9 ) : 2 = 8
x2 = ( 7 - 9 ) : 2 = - 1 ( не подходит )
ОТВЕТ 8
Проверка
√ ( 7*8 + 8 ) = 8
√ ( 56 + 8 ) = 8
√ 64 = 8
8 = 8