Question

Afla numerele a,b,c stiind ca sunt verificate simultan relatiile:
a+b=12
b+c=13
a+c=15

Answer 1

a+b=12           => a+b=12     =>b-c=-3
a+c=15 /*(-1)   => -a-c=-15

b-c=-3     => 2b=10  =>  b=5  =>   c=8   =>   a=7
b+c=13  

Answer 2

$a+b+b+c+a+c= 12+13+15 \\ 2(a+b+c)=40 \\ a+b+c=40:2=20 \\ c=20-12=8 \\ b=20-15=5 \\ a=20-13=7$