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# Sum of n terms of a sequence

• Last Updated : 13 Aug, 2020

If nth term of a sequence is given by Tn=an3+bn2+cn+d where a, b, c, d are constants then sum of n terms.

`Sn = a*Σ(n3)+b*Σ(n2)+c*Σ(n)+Σ(d) `

where Σ represent summation.

Proof :
Let us find some of this series. So,

```Sn = T1+T2+T3+T4+ ... +Tn
T1 = a(1)3+b(1)2+c(1)+d
T2 = a(2)3+b(2)2+c(2)+d
T3 = a(3)3+b(3)2+c(3)+d
...
...
Tn = a(n)3+b(n)3+c(n)+d ```

```Sn
= T1+T2+T3+ ... +Tn
= a((1)3+(2)3+(3)3+ ...
+(n)3)+b((1)2+(2)2+(3)2+ ...
+(n)2)+c(1+2+3+ ... +n)+d(1+1+1+ ... +1) ```
`Sn = a*Σ(n3)+b*Σ(n2)+c*Σ(n)+dn `

Similarly if we have been nth term for any higher order or lower order term in the format.

```Tn = a1np+a2np-1+a3np-2++
... +apn1+ap+1```
```Sn = a1Σ(np)+a2Σ(np-1)+a3Σ(np-2)+
... +apΣ(n)+nap+1```

where p, a1, a2, …… are some constants.

Example :
nth term is given as,

`Tn = n2+n+1 `

Calculate Sn

Explanation :

```Sn = Σ(Tn)
Sn = Σ(n2)+Σ(n)+Σ(1)
Sn = (n(n+1)(2n+1))/6+n(n+1)/2+n ```

Because,

```Σ(n2) = (n(n+1)(2n+1))/6,
Σ(n) = (n(n+1))/2,
Σ(1) = n ```

Thus we can find sum of any sequence if its nth term is given. This article is helpful in finding time complexity of equations when time complexity is given as function of n and we have to find the time complexity for the whole algorithm.

 `#include ``using` `namespace` `std;``int` `main()``{``    ``int` `k = 3;``    ``int` `sum = 0;``    ``// finding sum of n^4 terms``    ``for` `(``int` `i = 1; i <= k; i++)``        ``sum += (i * i * i * i);``    ``// sum of first k natural numbers is k(k+1)/2.``    ``sum += (k * (k + 1)) / 2;``    ``// we can also use code to calculate it``    ``// for(int i=1;i<=k;i++)``    ``// sum+=i;`` ` `    ``// sum of constant term 1 for n times is n``    ``sum += k;``    ``// we can also use code to calculate it.``    ``// for(int i=1;i<=k;i++)``    ``// sum+=1;``    ``cout << sum << endl;``}`

Output :

`107 `

The given code above finds summation of sequence for 3 terms in which,

`Tn = n4+n+1 `

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