Question

Find the fourth proportional of:​(ii) $5,9$ and $45$.​

Answer 1

Let the fourth proportional be $x$.
$\Rightarrow$ $5:9=45:x$
[$\because$ Ratio of first two terms $=$ Ratio of last two terms]
$\Rightarrow$$\frac{5}{9}=\frac{45}{x}$

$\Rightarrow$ $x=\frac{45\times 9}{5}=81$
Hence, the fourth proportion is $81$.