Question

In an AP if d = 5. S9 = 72, then ‘a’ and a9.

pls answer fast​

Answer 1

The answer of a is -12 and a9 is 28.

Answer 2

Answer:

☞Formula : Sn = n/2 {2a+(n-1)d}

☞Given : d=5 , n=9 , s9=72

☞Answer :

72 = 9/2 { 2a+(9-1)5 }

144 = 9 {2a+40}

144 = 18a + 360

18a = 144-360

a = -216/18

☞ a = -12

a9 = a+8d = -12+8(5) = -12+40 = 28

☞A9 = 28

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