Question

factorise:  a² - 23a + 42

Answer 1

a² - 23a + 42  ...[42 = 21 × 2 and 21 + 2 = 23]

= a² - 21a - 2a + 42

= a(a - 21) - 2(a - 21)

= (a - 21)(a - 2)