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| 1. |
Find the sum of all 2 digit odd positive numbers |
| Answer» Two digit odd positive numbers are 11,13,15,17...........99 are in A.P.Here a = 11 and d = 2, tn= 99, n = ?Sum of the n terms = (n/2)[2a+(n -1)d]But tn = a + (n -1)d⇒ 99 = 11+ (n-1)2⇒ 99 -11 = (n-1)2⇒ 88/2 = (n-1)∴ n = 45.subsitute n = 45 in sum of the n terms we obtain⇒ s45 = (45/2)(2×11 + (45 -1)2)⇒ s45 = (45/2)(110)⇒ s45 = 45×55.⇒ s45 = 2475.∴sum of all two digit odd positive numbers = 2475. | |