tentukan panjang, lebar dan luas Bila diketahui kelilingnya 90 cm

Keliling = 2 × (p + l)
90 = 2 × (x + 3 + x + 2)
90 = 2 × (2x + 5)
90 = 4x + 10
80 = 4x
x = 5

panjang :
= x + 3
= 5 + 3
= 8 cm

lebar :
= x + 2
= 5 + 2
= 7 cm

Luas = p × l
= 8 × 7
= 56 cm²

jadi masukin rumus keliling
K=2(p+l)
90 = 2((x+3)+(x+2))
90= 2 ((x+x) + (3+2))
90 = 2 (2x) + 5)
90-5 = 2(2x)
85÷2= 2x
42.5 = 2x
2x = 42.5
x= 42.5÷2
x = 21.25