Question

The sum of the A. P. , 1+2+3+.....+10 is:
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Answer 1

$a = 1 \\ d = 1 \\ l = 10 \\ n = 10 \\ sum = \frac{n}{2} (a + l) \\ sum = \frac{10}{2} ( 1 + 10) \\ sum = 5 \times 11 = 55$