an equation for a circle with center (0,0) and radius is 8

Question

anonymous · Answer

x^2+y^2=8^2

anonymous · Answer

if centre is (a,b) and radius is r

equation is given by...
$$(x-a)^{2}+(y-b)^{2}=r^{2}$$

try it out with the above values!!1

anonymous · Answer

well the options are a)x^2+y^2=64 
or b)x^2+y^2=8

anonymous · Answer

you substitute the values in above equation and get answer.......

hero · Answer

Actually if the center is (0,0), then the equation of a circle is

x^2 + y^2 = r^2

anonymous · Answer

soooo would the answer be x^2+y^2= 64 ?

hero · Answer

Yup

anonymous · Answer

thanks :)

anonymous · Answer

okay so theres a circle, and the raduis = 7. and it says write an equation, would it be x^2+y^2=7^2 ?

hero · Answer

They didn't give you the coordinates of its center point?

hero · Answer

The equation of a circle is x^2 + y^2 = r^2 if and only if the the coordinates of its center point is (0,0).

anonymous · Answer

Okay yes, thank you so much ! :)

hero · Answer

But you didn't answer my question

anonymous · Answer

oh, yeeh the coordinates are (0,0)  the center

hero · Answer

Okay, well, then, yes, your equation is correct. You can either say: 

x^2 + y^2 = 7^2 

or

x^2 + y^2 = 49

anonymous · Answer

okay, and theres another circle, and the points are outside the circle, it says (6,1) how would I write that equation ?

hero · Answer

You can use the formula the other guy provided

anonymous · Answer

(x-a)^2+(y-b)^2=r^2 ?

hero · Answer

Yes, that :)

hero · Answer

Be careful with the negative

anonymous · Answer

so it would be (0-6)^2+(0-1)^2=r^2 ?

hero · Answer

I had a feeling you were going to do that. You don't replace x with zero by the way

anonymous · Answer

and when I solve it would be 37=r^2 which would then be r=6.1 ?

anonymous · Answer

Oh then what do I do ?

hero · Answer

No, you replaced x and y with zeroes which is forbidden.

hero · Answer

The center point is (a,b) not (x,y) in this case. In other words, you leave x and y alone

hero · Answer

Wait a sec...The points are OUTSIDE the circle? You should probably post or draw the picture.

hero · Answer

This is a different case if a point is outside the circle.

anonymous · Answer

ohh okay hold on

hero · Answer

Usually, the given formulas are used when a point is on the circle and you know the center point.

anonymous · Answer

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anonymous · Answer

something like that

hero · Answer

And (6,1) is not on the circle?

hero · Answer

Can you figure out what any one of the points I just drew are?
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