Question

If a parabolic reflector is 20 cm in diameter and 5 cm deep, then the focus is

• A. (0, 5)
• B. (5, 0)
• C. (0, -5)
• D. (-5, 0)

Answer 1

The correct option is B (5, 0)

Let equation parabola $y^2=4ax$

$Oc=5cm$

So, $x-$Cordinate of point $A=5$

$Ab=20$

$AC=\frac{1}{2}\times AB=10$

$A(5,10)$

$A$ Lie on the parabola it will satisfting the eqquation $y^2=4\times 9\times 5$

$9=5$

focus $=(0,10)$

$=(5,0)$