Question

find the following sum:1+2+3+..... +20=....?
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Answer 1

Answer:

4,30

Step-by-step explanation:

1+2+3+4+20

1+2=3  ,  3+4=7   + 20

(3+7)+20

10+20= 30

Answer 2

Answer:

Required sum is 210.

Step-by-step explanation:

Here we want to find value of 1+2+3+..... +20.

We know,

1+2+3+4+......+n $= \frac{n(n + 1)}{2}$

Where n is any natural numbers.

For example,

When,$n = 5$

So,

$1 + 2 + 3 + 4 + 5 = \frac{5(5 + 1)}{2} \\ = \frac{5 \times 6}{2} \\ = 5 \times 3 \\ = 15$

When,$n = 11$

So,

$1 + 2 + 3 + ...... + 11 = \frac{11 \times (11 + 1)}{2} \\ = \frac{11 \times 12}{2} \\ = 11 \times 6 \\ = 66$

When,$n = 30$

So,

$1 + 2 + 3 + ...... + 30 = \frac{30 \times (30 + 1)}{2} \\ = \frac{30 \times 31}{2} \\ = 15 \times 31 \\ = 465$

Here given,$n = 20$

So,

$1 + 2 + 3 + ...... + 20 = \frac{20(20 + 1)}{2} \\ = \frac{20 \times 21}{2} \\ = 10 \times 21 \\ = 210$

Know more about natural numbers:

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