Here the two terms of an AP is given you have to find the sum of first n term of the AP
This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.
This is the Question Number 18 Of Exercise 11 C of RS Aggarwal Solution
Deepak BoraNewbie
If the 5th and 12th terms of an AP are -4 and -18 respectively, find the sum of first 20 terms of the AP.
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t5 = – 4
t12 = – 18
so,
a+4d = – 4………1)
a+11d = – 18…….2)
on subtracting 1 from 2 we have:
7d = – 14
d = – 2
on putting value of d in eq 1 we have:
a = 4
now,
S20 = 20/2{2a+(20-1)d}
= 10{8-38}
= 10 {-30}
= – 300