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Find the sum of first 8 terms of an arithmetic progression whose common difference is -4 and 7th term is 9.

Find the sum of first 8 terms of an arithmetic progression whose common difference is -4 and 7th term is 9. Correct Answer 152

GIVEN:

Common difference = -4 and 7th term = 9.

CONCEPT:

Arithmetic progression formulas for calculating nth term and sum of n terms.

FORMULA USED:

Sum of ‘n’ terms of an arithmetic progression = n/2 ×

nth term of an arithmetic progression =

Where

a = first term, d = common difference, n = number of terms

CALCULATION:

7th term = 9

⇒ a + (7 – 1) × -4 = 9

⇒ a – 24 = 9

⇒ a = 33

Hence,

⇒ Sum of first 8 terms = 8/2 ×

= 4 ×

= 152

∴ the Sum of first 8 terms is 152.

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