an ap 5,12,19.... has 50 term find its last term hence find the sum of first 20 terms of an a.p
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Solution :-
a = 5
d = 12 - 5 = 7
n = 50
an = a + (n - 1)d
an = 5 + (50 - 1)7
an = 5 + 49*7
an = 5 + 343
an = 348
Sum of the last 25 terms -
a = 348, d = - 7, n = 15
Sn = n/2[2a + (n - 1)d]
Sn = 15/2[2*348 + (15- 1) - 7]
Sn = 7.5[696 + (14 × - 7)]
Sn = 7.5[696 - 98]
Sn = 7.5*598
Sn = 4485
Sum of the last 15 term is 4485
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