Question

the nth triangular number is defined to be the sum of the first n positive integers.For example the 4th triangular number is 1+2+3+4=10. In the first 100 terms of the sequence 1,3,6,10,15,21,28,.... of triangular numbers how many are divisible by 7?

Answer 1

Answer:

1+2+3+4=10

1,3,6,10,15,,21,28