Question

Find fourth proportional to $13,17,39$

Answer 1

Let $x$ be the fourth proportional to $13,17,39$

$13:17::39:x$

$\frac{13}{17}=\frac{39}{x}$

$13\times x=17\times 39$

$x=\frac{17\times 39}{13}$

$x=17\times 3$

$x=51$

$\therefore 51$is fourth proportional to $13,17,39$.