Question

If 1+2+3+....+n
$1 + 2 + 3 + ... + n = 1 20 \:|$
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Answer 1

Answer:

n = 15th term or 15.

Step-by-step explanation:

1 + 2 + 3 + 4 + ... + n forms an AS, where 1 is a(first term) and '2-1 = 1' is d(common difference).

    Using, S = (n/2)(a + l),  l is last term.

Here, last term is n.  So,  

⇒ (n/2)(1 + n) = 120

⇒ (n + n^2)/2 = 120

⇒ n^2 + n = 240

⇒ n^2 + n - 240 = 0

⇒ n^2 + 16n - 15n - 240 = 0

⇒ n(n + 16) - 15(n + 16) = 0

⇒ (n + 16)(n - 15) = 0

           n = 15