perhatikan gambar disamping!! jika luas persegi panjang pqrs = 24 cm maka kelilingnya adalah

L = p x l
24 = (2x+2) (x +2)
24 = 2x² + 4x + 2x + 4
2x² + 4x + 2x + 4 -24 = 0
2x² + 6x - 20 = 0
2x² + 10x - 4x - 20 = 0
2x(x+5) -4 (x+5)
(2x-4) (x+5)
2x = 4 x = -5
x = 4/2 = 2

p = 2x + 2
= 2(2) + 2 = 4 + 2 = 6
l = 2+2 = 4
K = 2(p+l)
K = 2(6+4)
= 2 . 10 = 20 cm