Question

. Find the sum of 1 +2 +3 +…80

Answer 1

EXPLANATION.

Sum = 1 + 2 + 3 + . . . . . + 80.

As we know that,

Formula of :

Sum of first n natural number = ∑n = [n(n + 1)]/2.

Using this formula in the equation, we get.

⇒ (80)(81)/2 = 40 x 81 = 3240.

                                                                                                                     

MORE INFORMATION.

Some important results.

(1) = Sum of first n natural numbers = ∑n = [n(n + 1)]/2.

(2) = Sum of first n odd natural number = ∑(2n - 1) = n².

(3) = Sum of first n even natural number = ∑2n = n(n + 1).

(4) = Sum of squares of first n natural number = ∑n² = [n(n + 1)(2n + 1)]/6.

(5) = Sum of cube of first n natural number = ∑(n)³ = [n(n + 1)/2]².