JAMB Mathematics 2015 Paper

Concept:To factorise a quadratic expression like $x^2 + 9x + 20$, find two numbers whose product equals the constant term and whose sum equals the coefficient of $x$.Explanation:For $x^2 + 9x + 20$, we need two numbers that multiply to $20$ and add up to $9$.The numbers are $5$ and $4$ because $5 \times 4 = 20$ and $5 + 4 = 9$.Rewrite the expression by splitting the middle term: $x^2 + 5x + 4x + 20$.Group the terms: $(x^2 + 5x) + (4x + 20)$.Factor each group: $x(x + 5) + 4(x + 5)$.Now factor out the common binomial $(x + 5)$: $(x + 5)(x + 4)$.Answer:The factorised form is $(x + 5)(x + 4)$. Therefore, the correct option is B.