A) x² - 10x + 21 = 0
D = b² - 4ac = 100 - 4 × 21 = 100 - 84 = 16 = 4²
x1 =( 10 + 4) / 2 = 7
x2 = ( 10 - 4) / 2 = 3
Разложим по формуле:
a( x - x1)( x - x2)
( x - 7)( x - 3)
б) 5y² + 9y - 2 = 0
D = b² - 4ac = 81 - 4 × 5 × (-2) = 81 + 40 = 121 = 11²
y1 = ( - 9 + 11) / 10 = 0,2
y2 = ( - 9 - 11) / 10 = - 2
5( y - 0,2)( y + 2)