Let y varies directly as x . If y=12 when x=4, then write a linear equation. value of y when x=5 ?

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4. Linear Equations in Two Variables

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Let $y$ varies directly as $x$. If $y=12$ when $x=4,$ then write a linear equation. What is the value of $y$ when $x=5 ?$

Option A

Option B

Option C

Option D

Solution

$y$ varies of directly as $x.$

$\Rightarrow \quad y \propto x,$

$\therefore \quad y=k x$

Substituting $y =12$ when $x =4,$ we get

$12=k \times 4 \Rightarrow k=12 \div 4=3$

Hence, the required equation is $y =3 x$

The value of $y$ when $x=5$ is $y=3 \times 5=15$.

Standard 9

Mathematics

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