Question

18. Calculate the sum:
5 + (-5) + 5 + (-5) + ....5...
(i) if the number of term is 10.
(ii) If the number of terms is 11.​

Answer 1

HeRe Is Your Ans ⤵

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If N = 10

$sum = \frac{n}{2} (2a + (n - 1)d) \\ \\ = > sum = 5 × 10 + (9)( - 5) \\ \\ = > sum = 5 × 10 - 45 \\ \\ = > sum = - \: 5$

If N = 11

$sum \: = \frac{11}{2} (10 + (10)( - 5) \\ \\ sum = > \frac{11}{2} (10 - 50) \\ \\ sum = > \frac{11 \times ( - 40)}{2} \\ \\ sum = > \frac{ - 440}{2} \\ \\ sum = > 220$

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