Răspuns :
Răspuns:
a)
3 + 6 + 9 + ... + 60 =
= 3 × ( 1 + 2 + 3 + ... + 20 )
= 3 × 20×21/2
= 3 × 10 × 21
= 10 × 63
= 630
b)
5 + 10 + 15 + ... + 80 =
= 5 × ( 1 + 2 + ... + 16)
= 5 × 16 × 17 / 2
= 5 × 8 × 17
= 680
Succes! ♡︎♥︎♡︎
a) 3•(1+2+3+4+…+20)
S=3• (20•21:2)
S=3•(10•21)
S=3•210
S=630
b)5•(1+2+3+…+16)
S=5•(16•17:2)
S=5•(8•17)
S=5•136
S=680