Question

3x^2+42x+72 Student 1 Student 2 (X+2)(x+12). 3(x+2)(x+12) Which answer is the correct answer? What was wrong with that is incorrect? Explain what they did wrong. What important note can you make about factoring that you’ve learned from mistakes made on this problem?

Answer 1

3(x + 12)(x + 2)

Explanation:

Given

3x² + 42x + 72 ← factor out 3 from each term

= 3(x² + 14x + 24) ← factor the quadratic

Consider the factors of the constant term (+ 24) which sum to give the coefficient of the x- term.

The factors are + 12 and + 2, since

12 × 2 = 24 and 12 + 2 = 14, thus

x² + 14x + 24 = (x + 12)(x + 2) and

3x² + 42x + 72

= 3(x + 12)(x + 2) ← in factored form