Question

Multiply and then evaluate:

(4x + y) and (x – 2y); when x = 2 and y = 1.

Answer 1

(4x + y) × (x − 2y)

= 4x (x − 2y) + y (x − 2y)

= 4x² − 8xy + xy − 2y²

= 4x² − 7xy − 2y²

Verification:

When x = 2, y = 1

L.H.S. = (4x + y) (x − 2y)

= (4 × 2 + 1) (2 − 2 × 1)

= (8 + 1) (2 − 2)

= 9 × 0

= 0

R.H.S. = 4x² − 7xy − 2y²

= 4 (2)² − 7 × 2 × 1 − 2 (1)²

= 4 × 4 − 14 − 2

= 16 − 16

= 0

∴ L.H.S. = R.H.S.