100≤15a+11≤999
100-11≤15a≤999-11
89≤15a≤988
5,9(3)≤a≤65,8(6)
a∈{6,7,8,....65}
nr :{101,116,.....986}
S=101+116+...+986=1087×886/2=481541
nr termeni 986-101+1=886
ii grupam cate 2 si vom avea 886/2=443 sume egale cu 101+986=1087
formula progresie matematica
S=(101+986)×886:2=481541