Question

Given p(x) = x4 + x3 - 13x2 - 25x - 12

Answer 1

Answer + Explanation + Theory

When a number is divided by a number it results in a quotient and a remainder

E.g. 9 / 4 = 2 remainder 1

9 is the dividend
4 is the divisor
2 is the quotient
1 is the remainder

Same way when a polynomial is divided by a linear expression

E.g.

Ax^2 + bx + c / (x-b) = (x+a) + r

Which can be rearranged to

ax^2 + bx + c = (x+a)(x-b) + r

When x = - a or b, only the remainder is left since either (x+a)(x-b) is 0.

If x = - a or b is substituted into the polynomial and the value is 0 then there is no remainder,

This would suggest (x+a) or (x-b) are factors of the polynomial.

Now apply this logic to these questions

1. Let’s assume (x-4) is a factor, this would mean that when x=4 is substituted into the polynomial the answer would be 0.

This is the case, therefore the remainder is 0.

2. Having seen the logic above (applied using the remainder and factor theorem) the linear expression is a factor of the polynomial.