Question

Factorised form of r² – 10r + 21 is ______.

Options

• (r – 1)(r – 4)

• (r – 7)(r – 3)

• (r – 7)(r + 3)

• (r + 7)(r + 3)

Answer 1

Factorised form of r² – 10r + 21 is (r – 7)(r – 3).

Explanation:

We have, r² – 10r + 21

= r² – 7r – 3r + 21

= r(r – 7) – 3(r – 7)  ...[By splitting the middle term, so that the product of their numerical coefficients is equal constant term]

= (r – 7)(r – 3)   ...[∵ x² + (a + b)x + ab = (x + a)(x + b)]