a) a+a+1+a+2+a+3+...+a+14 = 30315
15a+1+2+3+...+14 = 30315
[tex]15a+\frac{14*15}{2} = 15a+7*15 = 15a+105[/tex]
15a+105 = 30315
15a = 30210
a = 30210:15 = 2014
b) 1+3+5+...+99 =
- Mai intai aflam al catelea termen din sir este 99
(99-1):2+1 = 98:2+1 = 49+1 =50
[tex]\frac{(1+99)*50}{2} = \frac{100*50}{2} = 50*50 = 2500[/tex]
