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

• 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.
• 10 years agoHelpfull: Yes(4) No(0)
• apply sn=n/2(2a+(n-1)d)
  and get the numbers -14 and 10...
  so 10 is the positive value
• 10 years agoHelpfull: Yes(0) No(0)
• 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
• 10 years agoHelpfull: Yes(0) No(0)
• 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
  
• 10 years agoHelpfull: Yes(0) No(0)