find the sum of all natural number for 40 to 400 which are divisible by 7
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Answer:
Sn = 11466
Step-by-step explanation:
42 , 49 , 56 ................ 399
a = 42
d = 7
n = ??
Sn = ??
an = a + (n - 1)d
399 = 42 + (n - 1) 7
399 = 42 + 7n - 7
399 = 7n + 35
7n = 364
n = 52
Sn = n [2a + (n - 1) d]
2
Sn = 52 [2*42 + (52 - 1) 7]
2
Sn = 26 [84 + 357]
Sn = 26 * 441
Sn = 11466
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