Question

what is the fourth proportional to 3,9,21

Answer 1

let the 4th proportion be x.

now:-
3:9: :21:x
3/9=21/x
x=(9X21)/3
x=189/3
x=63

Hence 4th proportion is 63.

Answer 2

The fourth proportional to 3, 9, 21 is 69

Step-by-step explanation:

To find the fourth proportional of 3, 9, 21, let the fourth proportional be x. therefore the value of 3, 9, 21, and x

We can write the number in the ratio is,

$a : b : : c : d=3 : 9=21 : x$

$\frac{3}{9}=\frac{21}{x}$

Now find the value of x by cross multiplying,  

$x \times 3=21 \times 9$

On rearranging, we get,

$x=\frac{189}{3}$

Therefore, we get,

$x=63$

Hence, the fourth proportional is 69