the sum of first 13 terms of a.p. is 21 and the sum of first 21 terms is 13 find the sum of the first 34 terms
Answers
Answered by
1
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
[tex]21=(f+6d)13 \\ 21=13f+78d...(1) \\ 13=21(f+10d) \\ 13=21f+210d ...(2) \\ [/tex]
we want to make f equal to calulate this eqution
[tex](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 [/tex]
sn=n*center term
s13=13* X7
21=13* X7
s21= 21* X11
13= 21* X11
x7=f+6d
x11=f+10d
[tex]21=(f+6d)13 \\ 21=13f+78d...(1) \\ 13=21(f+10d) \\ 13=21f+210d ...(2) \\ [/tex]
we want to make f equal to calulate this eqution
[tex](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 [/tex]
Similar questions