3. The first and last terms of an AP are 7 and 49 respectively. If sum of all
its terms is 420, find its common difference.
(3)
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Step-by-step explanation:
first term of AP = a = 7
last term of AP = l = a + (n - 1)d = 49
sum of the terms when last term is known
Sn = n/2 (a + l)
420 = n/2 (7 + 49)
420 = n/2 (56)
420 = 28n
n = 420/28
n = 15
l = a + (n-1)d = 49
7 + (15 - 1)(d) = 49
7 + 14d = 49
14d = 49 - 7
14d = 42
d = 42/14
d = 3
therefore the common difference = 3
Hope you get your answer
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