In an A.P. :Given a = 2, d = 8, Sn = 90, find ‘n’ and an f

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233.

In an A.P. :
Given a = 2, d = 8, Sn = 90, find ‘n’ and an


Here,    a = 2, d = 8, S= 90
We know that,

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#6 {main}</pre> [2 x 2 + (n -1)d]
rightwards double arrow     180  =n[2 x 2 (n - 1) x 8]
rightwards double arrow     180 = n(8n - 4)
rightwards double arrow     180 = 8n- 4n
rightwards double arrow     8n- 4n - 180 = 0
rightwards double arrow     2n- n - 45 = 0     [Dividing by 4]
rightwards double arrow    2n(n - 5) (2n + 9) = 0
rightwards double arrow    n - 5 = 0  or 2n + 9 = 0

rightwards double arrow    n = 5 or n = fraction numerator negative 9 over denominator 2 end fraction
But number of terms cannot be (- ve)

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#6 {main}</pre>
Now,   a= a+ 4d
              = 2 + 4 + 8
Hence,     n = 5      
a= 34

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