Question

When to use the formula an = sn - sn-1 and an= a + (n-1)d ?

Answer 1

any where anytime . use in any AP problum

Answer 2

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Here is your answer
when in AP
Sum is to be given or find out , use sn - sn- 1

When first term a and common difference d is given then n term can be find by
an = a + ( n- 1)d


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