Question

If x^2-13x+42=(x+A)(x+B), then find the value of 3A-B

Answer 1

Answer:

- 11 or - 15

Step-by-step explanation:

Factorise

x² - 13x + 42

Consider the factors of the constant term (+ 42) which sum to give the coefficient of the x- term (- 13)

The factors are - 6 and - 7, since

- 6 × - 7 = + 42 and - 6 - 7 = - 13, thus

x² - 13x + 42 = (x - 6)(x - 7)

compare to (x + A)(x + B)

A = - 6, B = - 7

thus 3A - B = 3(- 6) - (- 7) = - 18 + 7 = - 11

The factors may also be expressed as (x - 7)(x - 6)

with A = - 7 and B = - 6, then

3A - B = 3(- 7) - (- 6) = - 21 + 6 = - 15