n = 71 + 72 + 73 + ... + 123 progresie aritmetica
an=123, a1=71
an=a1+(n-1)×r
123=71+(n-1)×1
123-71=n-1
52+1=n
n=53
S=(71+123)×53):2=5141
100n=514100 rotunjit la zeci de mii
Rotunjiti la zeci de mii prin adaos 514000
b) n = 87 + 88 + 89 + ... + 157
an=157
a1=87
157=87+(n-1)×1
157-87+1=n
n=71
S=[(87+157)×71]:2=8662
100n=866200
Rotunjiti la zeci de mii produsul 876200
