Question

Let there be an A.P. with first term 'a', common difference 'd'. If an denotes its nth term and Sn the sum of first n terms, then find a, if d=-3, n= 12 and Sn=-126​

Answer 1

Answer:

a = 93/2

Step-by-step explanation:

We know,

Sₙ = [2a + (n - 1)d]

=> -126 = [2a + (12 - 1)(-3)]

=> -126 = 2a + 11(-3)

=> -126 = 2a - 33

=> 2a = -126 + 33

=> 2a = 93

=> a = 93/2

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Answer 2

Answer:

a=6

Step-by-step explanation:

Sn= -126,   n=12,   d=-3

Sn = n/2 [ 2a + (n-1) d ]

-126 = 12/2 [2a + (12 - 1) -3]

-126 = 6 [2a - 33]

-126/6 = 2a - 33

-21 + 33 = 2a

12 = 2a

a =6