Question

A rectangle has a length of 40 cm and width of 21 cm. If a new rectangle is formed by adding the same amount to both the length and width the new area is 2262 cm. Find the dimensions of the rectangle.

Answer 1

Explanation:

the area of a rectangle is

length × width.

so, the original area is

40 × 21 = 840 cm²

now, we add x to length and width, and we get :

(40 + x) × (21 + x) = 2262 cm²

let's do the multiplication :

40×21 + 40x + 21x + x² = 2262

840 + 61x + x² = 2262

x² + 61x = 1422

x² + 61x - 1422 = 0

a quadratic equation

ax² + bx + c = 0

has the general solution

x = (-b ± sqrt(b² - 4ac))/(2a)

in our case

a = 1

b = 61

c = -1422

x = (-61 ± sqrt(61² - 4×1×-1422))/(2×1) =

= (-61 ± sqrt(3721 + 5688))/2 =

= (-61 ± sqrt(9409))/2 =

= (-61 ± 97)/2

x1 = (-61 + 97)/2 = 36/2 = 18

x2 = (-61 - 97)/2 = -158/2 = -79

x2 would make the side lengths negative, and that does not make any sense for an actual shape, so

x = 18

is our valid solution.

that means the new length is

40 + 18 = 58 cm

the new width is

21 + 18 = 39 cm