Notam laturile cu litere:
a+b+c=42cm
a+b=c+12
b=a-7 => a=b+7
Inlocuim in relatie in functie de 'C'
c+12+c=42
2 x c+12=42cm
2 x c=42cm -12cm
2 x c=30cm
c=30 cm :2
c=15 cm
a+b=15 cm +12cm=27cm
a=b+7
Inlocuim in functie 'B'
b+7cm+b=27cm
2 x b=27cm-7cm=20cm
b=20cm :2
b=10cm
a=10cm+7cm
a=17cm
