Writing formulae

"Mugs cost £8 each plus £4 delivery for the order": n mugs cost 8n, and the delivery is added once, so C = 8n + 4.

Check with a number: 3 mugs should cost £28, and 8 × 3 + 4 = 28.

Keep the units the same: £3 plus 45p a minute, in pounds, is C = 3 + 0.45m.

A rectangle 3x long and y wide has perimeter P = 3x + y + 3x + y = 6x + 2y, and area A = 3xy.

Perimeter adds sides; area multiplies.

A frame round a square photo of side x, adding 2 cm each side: the frame area is (x + 4)² − x² = 8x + 16.