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The Brainliest Answer!
2015-02-14T14:02:08+05:30
Here the n are odd numbers
sn=n*center term
s13=13* X7
21=13* X7
s21= 21* X11
13= 21* X11
x7=f+6d
x11=f+10d
21=(f+6d)13 \\ 
21=13f+78d...(1) \\ 
13=21(f+10d) \\ 
13=21f+210d ...(2) \\ 

we want to make f equal to calulate this eqution
 \\

(21F+210D)13=13*13 \\ 
273F+2730D=169...(3) \\ 
21*13=(13F+78D)21 \\ 
273=273F+1014D...(4) \\ 

 \\  \\ 
273=273F+1014D- \\ 
169=273F+2730D
 \\ -1716D=104


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