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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This answer was edited.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