48

Penjelasan dengan langkah-langkah:

p x l = L

(4x - 12)(x + 7) = 800

4(x - 3)(x + 7) = 800 |:4|

(x - 3)(x + 7) = 200

x² + 7x - 3x - 21 = 200

x² + 4x - 21 = 200

x² + 4x - 21 - 200 = 0

x² + 4x - 221 = 0

(x + 17)(x - 13)

x + 17 = 0 atau x - 13 = 0

x = -17 x = 13

panjang

x = - 17

p = 4.(-17)-12 = -68 - 12 = -80 cm

x = 13

p = 4(13) - 12 = 52 - 12 = 40 cm

lebar

x = -17

l = -17 + 7 = - 10 cm

x = 13

l = x + 7 = 13 + 7 = 20 cm

karena p dan lebar tidak mungkin negatif maka yang memenuhi syarat p = 40 cm dan l = 20 cm

banyak lampu hias

= K : jarak

= 2(40+20) : 2,5

= 2 x 60 : 2,5

= 120 : 2,5

= 48