diketahui keliling persegi panjang 80 cm. jika perbandingan panjang lebar =7:3 luas persegi panjang adl... tlg pke cara

misal
p = 7x
l = 3x
k = 2 × (p + l)
80 = 2 × (7x + 3x)
80 = 2 . 10x
80 = 20x
x = 4
jadi p = 7x = 7.4 = 28
l = 3x = 3 .4 = 12
Luas = p × l = 28 × 12 = 336 cm²

Bab Bangun Datar
Matematika SMP Kelas VII

p : l = 7 : 3
p = 7l/3

Keliling = 80
2 . (p + l) = 80
7l/3 + l = 80 : 2
7l/3 + 3l/3 = 40
10l/3 = 40
l = 40 . 3/10
l = 12 cm

p = 7l/3
   = 7 . 12/3
   = 28 cm

Luas = p . l
         = 28 . 12
         = 336 cm²