Question

sn = 3n²-5n then a12=​

Answer 1

Answer:

$\large\boxed{\sf{55}}$

Step-by-step explanation:

Given that

$S{n} = 3 {n}^{2} - 5n$

Therefore, we have

=> a1 + a2 + a3 + ........ n terms = 3n²-5n

Now, for n = 1

=>a1 = 3×1- 5×1 = 3 - 5 = -2

Now for n = 2

=>a1 + a2 = 3×4 - 5×2 = 12-10 = 2

=> -2 + a2 = 2

=> a2 = 2+2

=> a2 = 4

Therefore, common difference = d

=> d = 4-(-2)

=> d = 4+2

=> d = 6

.°. a12 = a1+(n-1)d = a1 + 11d

=> a12 = -12 + (11×6)

=> a12 = -12 + 66

=> a12 = 55

Hence, a12 = 55