Arithmetic Progression
Algebra
Level
2
The first term of an arithmetic progression is -8.
Find the sum of the arithmetic progression up to 12 terms if the second term is -4.
The answer is 168.
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1 solution
Given,
a = − 8 d = − 4 − ( − 8 ) = 4
So, using S n = 2 n { 2 a + ( n − 1 ) d }
S 1 2 = 2 1 2 { 2 ( − 8 ) + ( 1 2 − 1 ) ( 4 ) } = 6 { − 1 6 + 4 4 } = 6 × 2 8 = 1 6 8