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(#M40024436) INFOSYS Numerical Ability question Keep an EYE

Find sum of the series 1-2+3-4+....-98+99

Asked In Infosys asxa asxaxax (11 years ago)

Unsolved Read Solution (9)

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Read Solution (9) :

Rupali Rastogi  (11 years ago)

ans z 50.v can write as-
(1+3+.....+99)-(2+4+....+98)ie 50 terms+49 terms resp.
using formla s=n/2[a+l]
we get, 50/2[1+99]-49/2[2+98]
den 2500-2450=50.

Is this solution Helpfull? Yes (19) | No (2)  

varaprasad  (11 years ago)

its simple,
lets take first 5 no's,
1-2+3-4+5=3=(middle no. or (5+1)/2=3)
lets take first 7 no's,
1-2+3-4+5-6+7=4=(middle no. or (7+1)/2=4)
lets take first 11 no's,
1-2+3-4+5-6+7-8+9-10+11=6=(middle no. or (11+1)/2=6)
so,
1-2+3-4+5-6+....-98+99 = (99+1)/2 = 50
ans:- 50

Is this solution Helpfull? Yes (10) | No   

ranjini  (9 years ago)

(1-2)+(3-4)+(5-6)+.....+(97-98)+99
(-1)+(-1)+......(-1)+99
-49+99
50

Is this solution Helpfull? Yes (4) | No   

Arpit Varshney  (11 years ago)

answer is -49+99=50

Is this solution Helpfull? Yes (2) | No (3)  

Divya Nimavat  (11 years ago)

Ans is = 99
sum of odd series = 1+3+5+...+99 = n^2=99^2
sum of even series = 2+4+6+...+98= n(n+1)=98(99)
so sum = 99^2-98(99)=99

Is this solution Helpfull? Yes (1) | No (3)  

darshan  (11 years ago)

ans.1
(1+3+5+...)-(2+4+..)
25(2+49)-49(4+48)/2
=1275-1274=1

Is this solution Helpfull? Yes | No (1)  

ruchi pandey  (10 years ago)

by n/2(a+l)
25(1+99)-49/2(2+98)
25*100-49*50=2500-2450
=50

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Ramkumar R R  (8 years ago)

(1-2) + (3-4) ........ + (97-98) = -1 + -1 ......... + -1 like that pairing upto -98 = -49
then -49 + 99 = 50

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karthick.G  (7 years ago)

Ans is -50
1+99 = 100
-2+(-98) = -100
3+97=100
-4+(-96)=-100

similarly,
49+51=100

hence ans is -50, because all even numbers are negative as per series.

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