. 9th term of arithmetic sequence is99 , and 99th term is 9. Find its common difference
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Let a and d be the first term and the common difference respectively of an ap.
(a.n)=a+(n-1)d
Given 9th term=99
a+8d=99--------------(1)
Given 99th term=9
a+98d=9-------------(2)
Subtract 1 from 2
a+98d-(a+8d)=9-99
a+98d-a-8d=-90
90d=-90
d=-90/90
d=-1
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