4.
а)
2*(3х-2у)(3х+2у) = 2*(9х²-4у²) = 18х² - 8у²,
б)
(4у²+9)(2у-3)(2у+3) = (4у²+9)(4у²-9) = 16у⁴ - 81,
в)
(7m²-3n³)(7m²+3n³) = 49m⁴ - 9n⁶,
5.
а)
25у² - 16 = 0,
25у² = 16,
у² = 16/25,
у = ±√(16/25),
у1 = 4/5, у2 = -4/5, (или у1 = 0,8, у2 = -0,8),
б)
(х - 2)(х + 2) - х(х + 5) = -8,
х² - 4 - х² - 5х = -8,
5х = 8 - 4,
5х = 4,
х = 4/5 (или х = 0,8)