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The point (7, 4) lies on a circle centered at the origin. Write the equation of the circle and state the radius.

Sagot :

Given:

The point (7, 4) lies on a circle centered at the origin.

To find:

The equation of the circle and the radius of the circle.

Solution:

Distance formula:

[tex]d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]

The point (7, 4) lies on a circle centered at the origin. So, the distance between the points (7,4) and (0,0) is equal to the radius of the circle.

[tex]r=\sqrt{(0-7)^2+(0-4)^2}[/tex]

[tex]r=\sqrt{(-7)^2+(-4)^2}[/tex]

[tex]r=\sqrt{49+16}[/tex]

[tex]r=\sqrt{65}[/tex]

The standard form of the circle is

[tex](x-h)^2+(y-k)^2=r^2[/tex]            ...(i)

Where, (h,k) is the center of the circle and r is the radius of the circle.

Putting h=0, k=0 and [tex]r=\sqrt{65}[/tex] in (ii), we get

[tex](x-0)^2+(y-0)^2=(\sqrt{65})^2[/tex]

[tex]x^2+y^2=65[/tex]

Therefore, the equation of the circle is [tex]x^2+y^2=65[/tex] and its radius is [tex]r=\sqrt{65}[/tex].