Question

The sum of n terms of the AP 5, 2, -1, -4, -7,.... is

Answer 1

✪ Given:-

• AP:-5,2,(-1),(-4),(-7),... , where a=5,d=(-3).

✪ To Find:-

   Sₙ (Sum of n terms)

✪ Formula Applied:-

• Sₙ=$\frac{n}{2}$[2a+(n-1)d]

✪ Solution:-

    Sₙ=$\frac{n}{2}$[2a+(n-1)d] , where a=5, d=(-3).

⇒ Sₙ=$\frac{n}{2}$[2(5)+(n-1)(-3)]

⇒ Sₙ=$\frac{n}{2}$(10-3n+3)

⇒ Sₙ=$\frac{n}{2}$(13-3n)

$\implies \large\boxed{S_n=\frac{13n-3n^2}{2} }$