Question

The sum of an AP is 1815. If the first and last terms are -12 and 133 respectively, find the number of terms in the AP

Answer 1

$s_{n} = \frac{n}{2} (first \: term \: \: \: + \: \: \: last \: term)$


Here,


$1815 = \frac{n}{2} ( - 12 + 133) \\ \\ \\ => 1815 = \frac{n}{2} (121) \\ \\ \\ => \frac{1815 \times 2}{121} = n \\ \\ \\ => 30 = n$


Hence, there are 30 terms.

Answer 2

here is your answer


n/2×(-12 + 133) = 1815

n/2×(121) = 1815

n= 1815×2/21 = 30



n=30 [ is right answer ]



Hope it helps you ^_^


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