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CBSE Class 10 Answered

Page 255 Question 10 If the sum of n terms of an AP is 3n2+3n, find the rth term of the series.
Asked by miniprasad | 08 Dec, 2018, 10:50: PM
answered-by-expert Expert Answer
rth term of AP = a + ( r - 1 )d 
 
To get rth term, we need two unknowns first term a and common difference d.
Hence we need two equations to get these two unknowns.
But we are given only one equation which is the expression given for sum of A.P
So we will eliminate one unknown a and express the rth term using other unknown b
 
Sum = begin mathsize 12px style n over 2 open parentheses 2 a space plus space open parentheses n minus 1 close parentheses d space close parentheses space equals space 3 n squared plus 3 n space equals space 3 n left parenthesis n plus 1 right parenthesis end style
hence     a = 3(n+1) - (1/2)(n-1)d
 
rth term = a +(r-1)d = 3(n+1) - (1/2)(n-1)d + (r-1)d = 3(n+1)+d[ r - (1/2)(n-1) ]
Answered by Thiyagarajan K | 08 Dec, 2018, 11:50: PM
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