Salutare!
a = 1 + 2 + 3 + 4 + ... + 23
a = 23 × ( 23 + 1 ) : 2
a = 23 × 24 : 2
a = 552 : 2
a = 276
b = 3 + 6 + 9 + 12 + ... + 84
b = 3 × ( 1 + 2 + 3 + 4 + ... + 28 )
b = 3 × [ 28 × ( 28 + 1 ) : 2 ]
b = 3 × ( 28 × 29 : 2 )
b = 3 × ( 812 : 2 )
b = 3 × 406
b = 1218
Am folosit formula sumei lui Gauss:
- 1 + 2 + 3 + ... + n = n × ( n + 1 ) : 2
