Latest Campus Aptitude Question SOLUTION: Fine the positive value of n when the sum of 5+7+9+……..=140.

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(#M40034263) CAMPUS Numerical Ability question simplification Keep an EYE

Fine the positive value of n when the sum of 5+7+9+……..=140.

Asked In Campus gaurav (10 years ago)

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Surya Narayana K  (10 years ago)

Ans.. n=10
Given 5+7+9+.....=140
Let first number(a)=5, last number=l and there are n numbers.
so, sum of given numbers (n/2)(a+l)=140=>5n+nl=140 ---(1)
and l=a+(n-1)d=5+(n-1)2=3+2n--(2)
Substitute (2) in (1),
5n+n(3+2n)=280 =>2n^2+8n-280=0
n^2+4n-140=0.
n=-14 or 10.
So there 10 positive numbers are there.

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anu joshi  (10 years ago)

apply sn=n/2(2a+(n-1)d)
and get the numbers -14 and 10...
so 10 is the positive value

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eswariteja  (10 years ago)

sum of n terms formula is s=n/2(2a+(n-1)d)
n=? ,s=140, a=5 ,d=2 the series in A.P.
n^2+4n+140=0;
n=14 or 10

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Ram Sah  (10 years ago)

the given serries in arithmetic serries with first term=5 n c.d.=2 so using formula sn=n(2a+(n-1)d)/2 we can value of n to be 10

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