Question

4a+3b+7c=24 7a+9b+c=42 a+b+c=?

Answer 1

4a+3b+7c=24
7a+9b+c=42
Adunand acestea obtinem:
11a+12b+8c=66
10a+a +10b+2b+10c-2c=66
(10a+10b+10c)+(a+2b-2c)=66  (1)

vom scadea( 7a+9b+c)-(4a+3b+7c)=3a+6b-6c=42-24
3a+6b-6c=18 ; vom imparti toata relatia la 3 
a+2b-2c=6 (2)
Si dupa cum observi mai sus ,in relatia (1) ,relatia (2) este chiar a doua paranteza 
Deci inlocuim acolo 
(10a+10b+10c )+(a-2b-2c)=66
10(a+b+c)+6=66
10(a+b+c)=66-6
10(a+b+c)=60 I :10
a+b+c=6