Fie L si l lungimea si latimea dreptungiului. Perimetru Abcd=AB+BC+AD+DC=L+l+l+L=220!(divide) :2 L+l=110 -daca marim latimea cu 10cm =l+10 ,daca am micsora lungimea cu 20 cm=L-20cm . Aria intiala=L x l Aria finala=(L-20)x(l+10) Aria initiala=Aria finala. L x l=(L-20)x(l+10) L x l =L xl +L x10 -20xl-20X10. L x l =Ll+10l-20l-200 . O =Ll+10L-20l-200-Lxl (se reduc Ll cu Ll) . 10L-20l-200=o . 10L=20l+200 ! :10 . L=2l+20