Question

7+15+23+31+...+103-3-7-11-15-...-51

Answer 1

7+15+23+31+...+103-3-7-11-15-...-51
7+15+23+31+...+103
pasul 15-7=8
nr de numere (103-7):8+1=13
mijlocul sirului  7+6*8=55
7+15+23+31+...+103=(7+103) (13-1)/2+55=110*6+55=715
-3-7-11-15-...-51
pasul 7-3=4
nr de numere (51-3):4+1=13
mijlocul sirului  3+6*4=27
-3-7-11-15-...-51 =(-3-51)*6-7=-54*6-7=-324-27=-351

7+15+23+31+...+103-3-7-11-15-...-51 =715-351=364