Find the smallest number in the progression 3, 12, 48, ... which is greater than 10 000.
Solution
Given Sequence : $ \displaystyle 3, 12, 48, ...$
$ \displaystyle \therefore \frac{{{{u}_{2}}}}{{{{u}_{1}}}}=\frac{{12}}{3}=4$
$ \displaystyle \ \ \ \frac{{{{u}_{3}}}}{{{{u}_{2}}}}=\frac{{48}}{{12}}=4$
$ \displaystyle \therefore \frac{{{{u}_{2}}}}{{{{u}_{1}}}}=\frac{{{{u}_{3}}}}{{{{u}_{2}}}}$
Therefore the given sequence is a geometric progression with the first term $ \displaystyle 3$ and the commratio $ \displaystyle 4$.
$ \displaystyle \therefore a=3$ and $ \displaystyle r=4$.
Let the smallest number in the progression which is greater than $ \displaystyle 10\ 000$ be $ \displaystyle {{{u}_{n}}}$
$ \displaystyle \therefore {{u}_{n}}>10\ 000$
$ \displaystyle \ \ a{{r}^{{n-1}}}>10\ 000$
$ \displaystyle \ 3({{4}^{{n-1}}})>10\ 000$
$ \displaystyle \ \ {{4}^{{n-1}}}>3333.33$
But $ \displaystyle {{4}^{5}}=1024<3333 .33$ and $ \displaystyle {{4}^{6}}=4096>3333 .33$
$ \displaystyle \therefore n-1=6$
$ \displaystyle \therefore n=7$
$ \displaystyle \therefore {{u}_{7}}=3({{4}^{6}})=12\ 288$
Therefore the smallest number in the progression which is greater than $ \displaystyle 10\ 000$ is $ \displaystyle 12\ 288$.
Solution
Given Sequence : $ \displaystyle 3, 12, 48, ...$
$ \displaystyle \therefore \frac{{{{u}_{2}}}}{{{{u}_{1}}}}=\frac{{12}}{3}=4$
$ \displaystyle \ \ \ \frac{{{{u}_{3}}}}{{{{u}_{2}}}}=\frac{{48}}{{12}}=4$
$ \displaystyle \therefore \frac{{{{u}_{2}}}}{{{{u}_{1}}}}=\frac{{{{u}_{3}}}}{{{{u}_{2}}}}$
Therefore the given sequence is a geometric progression with the first term $ \displaystyle 3$ and the commratio $ \displaystyle 4$.
$ \displaystyle \therefore a=3$ and $ \displaystyle r=4$.
Let the smallest number in the progression which is greater than $ \displaystyle 10\ 000$ be $ \displaystyle {{{u}_{n}}}$
$ \displaystyle \therefore {{u}_{n}}>10\ 000$
$ \displaystyle \ \ a{{r}^{{n-1}}}>10\ 000$
$ \displaystyle \ 3({{4}^{{n-1}}})>10\ 000$
$ \displaystyle \ \ {{4}^{{n-1}}}>3333.33$
But $ \displaystyle {{4}^{5}}=1024<3333 .33$ and $ \displaystyle {{4}^{6}}=4096>3333 .33$
$ \displaystyle \therefore n-1=6$
$ \displaystyle \therefore n=7$
$ \displaystyle \therefore {{u}_{7}}=3({{4}^{6}})=12\ 288$
Therefore the smallest number in the progression which is greater than $ \displaystyle 10\ 000$ is $ \displaystyle 12\ 288$.
စာဖတ်သူ၏ အမြင်ကို လေးစားစွာစောင့်မျှော်လျက်!