6.
a= 1+5 +5²+5³ +5⁴ +5⁵..+5²⁰¹³ divizibil cu 6.
a= 5⁰(1+5)+ 5²(1+5)+5⁴(1+5)....+5²⁰¹²(1+5)
a= (1+5)(5⁰+5²+....5²⁰¹²)
a=6*(5⁰+5²+....5²⁰¹²) |:6
7.
(x-3)*(y+2)=12
daca 12=3*4 daca 12=6*2
x-3=3; x=6 x-3=6; x=9
y+2=4; y=2 y+2=2; y=0
daca 12=4*3 daca 12=2*6
x-3=4; x=7 x-3=2; x= 5
y+2=4; y=2 y+2=6; y=4
daca 12=1*12 daca 12=12*1
x-3=1; x=4 x-3=12; x=15
y+2=12; y=10 y+2=1; y= - 1
8.
3a+2b=16 ; adunarea de doua numere pare, caci 16 =par si 3*nr par=par
singurul nr prim par este 2.
a=2; 3*2=6
6+2b=16
2b=10
b=5 ; a=2; b=5
