adunam relatiile : 4a+4b+4c=300 4(a+b+c)=300 a+b+c=300/4=75 scadem din relatia 2 pe prima : 3a+2b+c-a-2b-3c=-20 2a-2c=-20 2(a-c)=-20 a-c=-10 deci produsul lor : 75*(-10)=-750
adunam cele doua expresii si obtinem 4a+4b+4c=300, impartim la 4 =>a+b+c=75. scadem cele doua expresii 3a+2b+c-a-2b-3c=140-160 => 2a-2c=-20, impartim la 2 => a-c=-10. (a+b+c)*(a-c)=75* (-10)=-750
