Răspuns :
P = 2( L + l )
40 cm = 2( 12 + l )
40 cm = 2 × 12 + 2 × l
40 cm = 24 + 2 × l
2 × l = 40 - 24
2 × l = 16 cm
l = 16 : 2
l = 8 cm
40 cm = 2( 12 + l )
40 cm = 2 × 12 + 2 × l
40 cm = 24 + 2 × l
2 × l = 40 - 24
2 × l = 16 cm
l = 16 : 2
l = 8 cm