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# Find the sum of first 100 terms -5,-4,-3,-2,-1,0,1,2………using Gauss method.

Last updated date: 29th Mar 2023
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Hint: Gauss method is applicable only to arithmetic progression and from this we can conclude that the sequence is in AP and apply the suitable formula to find the sum of the given series.

The given series is -5,-4,-3,-2,-1,0,1,2………
We can conclude and say that this series is in Arithmetic Progression
Since, ${T_3} - {T_2} = {T_2} - {T_1}$ = -4-(-5)=1=d=common difference
We have to find out the sum of first 100 terms of the series using Gauss method
We know that Gauss formula is given by ${S_n} = \dfrac{n}{2}[first{\text{ }}term + last{\text{ }}term]$
In the given series the first term=-5, last term is unknown
So, let us find out the nth term (last term) by making use of ${T_n}$ formula of AP
We know that the nth term ${T_n}$ of an AP is given by ${T_n} = a + (n - 1)d$
Here a=-5, n=100, d=1
Let’s substitute these values in the formula
So, we get ${T_{100}}$ =-5+(100-1)1
=-5+99
${T_{100}}$ =94=last term
We have to find out the sum of the first 100 terms by Gauss formula
${S_n} = \dfrac{n}{2}[first{\text{ }}term + last{\text{ }}term]$
Here a=-5, d=1, last term=100
So, we can write
${S_{100}} = \dfrac{{100}}{2}[ - 5 + 94]$
=50[89]
=4,450
So, we can write ${S_{100}} = 4450$

Note: In this question ,we have been asked to find out the sum by Gaussian method , if it was not mentioned, we can find out the sum of the series by making use of an alternate formula.