2 + 6 + 10 + 14 + ...... + 30 = 2 × x³
2 × ( 1 + 3 + 5 + 7 + ..... + 15 ) = 2 × x³
1 + 3 + 5 + 7 + ..... + 15 = 8 × ( 1 + 15 ) : 2 = 4 × 16 = 64
→ stabilesc cati termeni are suma cu ratia 2
( 15 - 1 ) : 2 + 1 = 14 : 2 + 1 = 8 termeni are suma
→ aplic formula sumei lui Gauss
2 x³ = 2 × ( 1 + 3 + 5 + 7 + ..... + 15 )
2 x³ = 2 × 64 = 2 × 2⁶
2 x³ = 2 × 2⁶ l : 2
x³ = ( 2² ) ³ ⇒ x³ = 4³ ⇒ x = 4
