Find the fourth proportional to (i) \[\mathbf{3},\text{ }\mathbf{12},\text{ }\mathbf{15}\] (ii) \[\mathbf{1}/\mathbf{3},\text{ }\mathbf{1}/\mathbf{4},\text{ }\mathbf{1}/\mathbf{5}\] - Noon Academy

Find the fourth proportional to (i)

$\mathbf{3},\text{ }\mathbf{12},\text{ }\mathbf{15}$

(ii)

$\mathbf{1}/\mathbf{3},\text{ }\mathbf{1}/\mathbf{4},\text{ }\mathbf{1}/\mathbf{5}$

Class 10 | Exercise 1 | ICSE | Maths | ML Aggarwal | Ratio and Proportion

Find the fourth proportional to (i)

$\mathbf{3},\text{ }\mathbf{12},\text{ }\mathbf{15}$

(ii)

$\mathbf{1}/\mathbf{3},\text{ }\mathbf{1}/\mathbf{4},\text{ }\mathbf{1}/\mathbf{5}$

(i)

$\mathbf{3},\text{ }\mathbf{12},\text{ }\mathbf{15}$

Consider x as the fourth proportional to

$\mathbf{3},\text{ }\mathbf{12},\text{ }\mathbf{15}$

$3:\text{ }12\text{ }::\text{ }15:\text{ }x$

We can write it as

$3\text{ }\times \text{ }x\text{ }=\text{ }12\text{ }\times \text{ }15$

So we get

$\begin{array}{*{35}{l}} x\text{ }=\text{ }\left( 12\text{ }\times \text{ }15 \right)/\text{ }3 \\ x\text{ }=\text{ }60 \\ \end{array}$

(ii)

$1/3,\text{ }1/4,\text{ }1/5$

Consider x as the fourth proportional to

$1/3,\text{ }1/4,\text{ }1/5$

$1/3:\text{ }1/4::\text{ }1/5:\text{ }x$

We can write it as

$1/3\text{ }\times \text{ }x\text{ }=\text{ }1/4\text{ }\times \text{ }1/5$

So we get

$\begin{array}{*{35}{l}} x\text{ }=\text{ }1/4\text{ }\times \text{ }1/5\text{ }\times \text{ }3/1 \\ x\text{ }=\text{ }3/20 \\ \end{array}$

i

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