If nth term of a sequence is given by the formula an-10-3n, find the sum of 20terms.

• nth term = an-10-3n
  the sequence is in arithmetic progression with first term = a-13 d=a-3
  20th term= 20a-70
  sum of 20 terms 20/2(a-13 + 20a - 70)
  =210a - 830
• 9 years agoHelpfull: Yes(3) No(0)
• Tn=an-10-3n
  
  First term T1=a-10-3=a-13
  
  last(20th) term T20=20a-10-60=20a-70
  
  Sum of series(n terms)=n*(first term + last term)/2
  
  therefore Sum of 20 terms=20*(a-13+20a-70)/2
  =210a-830
  
  
  
• 8 years agoHelpfull: Yes(2) No(0)
• ans is -830.
  1st term = -10-3*1=-13
  2nd=-10-3*2=-16
  20th=-10-3*20=-70
  for AP sum of n no. is ,s=n/2(2a+(n-1)d)or n/2(a+last term)
  therefore 20/2(-13-70)=-830
• 8 years agoHelpfull: Yes(1) No(1)
• common difference here is, d= a-3
  and first term, a = a-13
  now using formula for sum of n terms of A.P = n/2[2a + (n-1)d]
  for n = 20
  We get answer = 210a - 830
• 9 years agoHelpfull: Yes(0) No(0)