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4. Linear Equations in Two Variables
easy
If the point $(3,5)$ lies on the graph of the equation $a x+y=20,$ find the value of $a$.
A
$3$
B
$7$
C
$5$
D
$9$
Solution
since every point on the graph of a linear equation is a solution of the equation and vice-versa, $x=3$ and $y=5$ is a solution of the equation $a x+y=20$.
$\therefore a(3)+5=20$
$\therefore 3 a+5=20$
$\therefore 3 a=20-5$
$\therefore 3 a=15$
$\therefore a=\frac{15}{3}$
$\therefore a=5$
Thus, the value of $a$ is $5 .$
Standard 9
Mathematics
