Розділ 3. КВАДРАТНІ РІВНЯННЯ » 21.7

Розв’яжіть рівняння: 1) x2 – 4x – 5 = 0; D = (–4)2 – 4 • 1 • (–5) = 36; √D = 6; x1 = (4+6)/2 = 5; x2 = (4-6)/2 = –1. 2) 2x2 + 7x – 4 = 0; D = 72 – 4 • 2 • (–4) = 81; √D = 9; x1 = (-7+ 9)/(2 •2) = 2/4 = 0,5; x2 = (-7- 9)/(2 •2) = (-16)/4 = –4. 3) 3x2 – 12x + 36 = 0; D = (–12)2 – 4 • 1 • 36 = 0; x1 = –(-12)/(2 ) = 6. 4) x2 – x – 56 = 0; D = (–1)2 – 4 • 1 • (–56) = 225; √D = 15; x1 = (1+15)/2 = 8; x2 = (1-15)/2 = –7.