Question

the common difference of AP 25,20,15.....is​

Answer 1

Common difference (d ) is given by

$d=2_{nd}\:term-1_{st}\:term$

$d=20-25=\bold{-5}$

Answer 2

Given,

a=25

d=20−25=−5

Tn<0

a+(n−1)d<0

25+(n−1)(−5)<0

25<5(n−1)

5<(n−1)

n>6

⇒n=7

7th term is negative.

a7 =25+(7−1)(−5)

=25−5×6

=−5

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