21, 28, 36, 45
Yes.
230
Starting with 1, it is 15.
Yes, the number 21 is indeed a triangular number. Other triangular numbers are 6,10, and 15.
21, 28, 36, 45
15 (1 + 2 + 3 + 4 + 5).
15, 21 (the triangular numbers)
The fifth triangular number is 15
Only 9 and 15 are odd so the rest can be rejected. 9 is not a triangular number so only 15 is odd and a triangular number.
Yes.
230
Starting with 1, it is 15.
The answer would be 15 and 21. The triangular numbers are 1,3,6,10,15,21,28,36,45,55,66,78. 15 plus 21 is 36 which is on the list. 21 minus 15 is 6 and that is on the list as well.
Yes, the number 21 is indeed a triangular number. Other triangular numbers are 6,10, and 15.
15
28 and 15