The sum of $n$ terms of two arithmetic progressions are in the ratio $(3 n+8):(7 n+15) .$ Find the ratio of their $12^{\text {th }}$ terms.
The sum of $n$ terms of two arithmetic progressions are in the ratio $(3 n+8):(7 n+15) .$ Find the ratio of their $12^{\text {th }}$ terms.
Let $a_{1}, a_{2}$ and $d_{1}, d_{2}$ be the first terms and common difference of the first and second arithmetic progression, respectively. According to the given condition, we have
$\frac{{{\rm{ Sum}}\,\,{\rm{to}}\,\,{\rm{nterms}}\,\,{\rm{of}}\,\,{\rm{ first }}\,{\rm{A}}{\rm{.P}}{\rm{. }}}}{{{\rm{ Sumt}}\,\,\,{\rm{on}}\,\,{\rm{ terms }}\,\,{\rm{of }}\,\,{\rm{second }}\,\,{\rm{A}}{\rm{.P}}{\rm{. }}}} = \frac{{3n + 8}}{{7n + 15}}$
or $\frac{\frac{n}{2}\left[2 a_{1}+(n-1) d_{1}\right]}{\frac{n}{2}\left[2 a_{2}+(n-1) d_{2}\right]}=\frac{3 n+8}{7 n+15}$
or $\frac{2 a_{1}+(n-1) d_{1}}{2 a_{2}+(n-1) d_{2}}=\frac{3 n+8}{7 n+15}$ .........$(1)$
Now $\frac{{{{12}^{{\rm{th }}}}{\rm{ term}}\,\,{\rm{of }}\,\,{\rm{first \,A}}{\rm{.P}}{\rm{. }}}}{{{{12}^{{\rm{th }}}}{\rm{ term}}\,\,{\rm{of }}\,\,{\rm{second \,A}}{\rm{.P }}}} = \frac{{{a_1} + 11{d_1}}}{{{a_2} + 11{d_2}}}$
$\frac{2 a_{1}+22 d_{1}}{2 a_{2}+22 d_{2}}=\frac{3 \times 23+8}{7 \times 23+15}$ [ By putting $n=23$ in $(1)$ ]
Therefore $\frac{a_{1}+11 d_{1}}{a_{2}+11 d_{2}}=\frac{12^{\text {th }} \text { term of first A.P. }}{12^{\text {th }} \text { term of second A.P. }}=\frac{7}{16}$
Hence, the required ratio is $7: 16$
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