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    				| 1. | Find the sum of all integers between 84 and 719, which are multiples of 5 | 
| Answer» The series is 85, 90, 95, …, 715 Let there be ‘n’ terms in the AP So, a = 85, d = 90-85 = 5, an = 715 an = a + (n - 1)d 715 = 85 + (n - 1)5 715 = 85 + 5n – 5 5n = 715 – 85 + 5 5n = 635 n = 635/5 = 127 By using the formula, Sum of n terms, S = n/2 [a + l] = 127/2 [85 + 715] = 127/2 [800] = 127 [400] = 50800 ∴ The sum of all integers between 84 and 719, which are multiples of 5 is 50800. | |