## Sum of Squares of First N Natural Numbers

To find the sum of squares of first N natural numbers, read the value of N from user using input(), then iterate from 1 to N using a For Loop, find square of each number and accumulate it in sum over the iterations.

The sum of squares of first N natural numbers is

`sum = 1`^{2} + 2^{2} + 3^{2} + . . . + n^{2}

If you are good in remembering formula or remembering that there is a formula, then the following will help you in finding the sum of squares of first `n`

natural numbers.

`n (n + 1) (2n + 1) / 6`

We shall go through two of these methods to find the sum of squares of first n natural numbers using Python.

## Program using For Loop

In the following program, we shall define a function `sumOfSquares()`

that takes `n`

as argument, use For Loop to compute the sum of squares, and then return the sum.

We read `n`

from user, call `sumOfSquares()`

with the `n`

passed as argument.

**Input**

`n`

, where `n > 0`

**Python Program**

```
def sumOfSquares(n) :
if n < 0:
return
sum = 0
for i in range(n+1):
sum += i*i
return sum
n = int(input('Enter n : '))
sum = sumOfSquares(n)
print(f'Sum : {sum}')
```

**Output**

```
Enter n : 6
Sum : 91
```

**Important Points regarding above program**

- input() function reads a string from standard input. Therefore, we have used int() function to convert string to integer.
- If given
`n`

is less than zero, we are returning nothing. - We used range(1, n+1) to iterate from 1 to n.
`+=`

is Addition Assignment Operator.

## Program using Formula

In the following program, we shall define a function `sumOfSquares()`

that takes `n`

as argument, uses the following formula to compute the sum of squares, and then return the sum.

`n (n + 1) (2n + 1) / 6`

We read `n`

from user, call `sumOfSquares()`

with the `n`

passed as argument.

**Input**

`n`

, where `n > 0`

**Python Program**

```
def sumOfSquares(n) :
if n < 0:
return
sum = n * (n + 1) * (2 * n + 1) / 6
return int(sum)
n = int(input('Enter n : '))
sum = sumOfSquares(n)
print(f'Sum : {sum}')
```

**Output**

```
Enter n : 6
Sum : 91
```

Since we have division operation in the formula, Python implicitly typecasts the value to floating-point number. We may convert the float to int using int() function.

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