Hello me and my friend have been trying to figure out whay is wrong with my ciricle formula. When completing a check to see if the formula is correct we get the wrong answer even though we should be edning up with the correct answer. It goes like this:

(the math done here is for my algbra two and trig class) 



Parent fucntion: x+ y= r

a= 9     b= 9 

h=1      k=4  

(x-1)+ (y-4)= 9 

Transformation: (x,y) = (9x+1), (9y+4) 

Equation: y= 4 plus or minus 9 square root 9-(x-1)2  



Point: (x,y) = (1,13) 

(1-1)2 + (y-4)2 = 9

0+ (y-4)= 9 

0 + square root (y-4)= square root 9 

y-4 = 3 

that would equal 7 which does not equal 13. However if I were not to square root 9 I would get 13, But that would be breaking the rules, Can anyone explain what I am doing wrong here? 

Guest Nov 26, 2017

I'm no expert at this, but notice that you have:


 x^2 + y^2 = r^2  as the "parent function", then further down you have this:


(x-1)^2 + (y-4)^2 = 9   Notice that you are squaring the LHS. Shouldn't you then square the RHS as well? So, when checking it you have this:


(1-1)^2 + (y-4)^2 = 9, but if you squared 9^2 on RHS, then you will have:

y =-5       and       y = 13, which is what you want.

Guest Nov 26, 2017

I am not squaring the right side of my equatin as It is already squared. I don't  need to put three squared unlike the left side where I need to do some steps before squaring. The right side is already solved. If you rerfer to the parent function x+ y= r2 you'll see that r is squared. Sorry for the confusion.

Guest Nov 26, 2017
edited by Guest  Nov 26, 2017

I believe I figured it out if you use (x-1)2 + (y-4)2 = 9 and plug in 1 for x and 13 for y you will get 3 = 3.

As the x part of the equation equals zero then 13-4 equals 9 sqeared. So you end up with 9= 9. Then just square root them and you get 3 equals 3. 

Guest Nov 26, 2017

I don't know what your  " a "  and  " b "  stand for, but it looks like you want the equation of a circle with its center at  (1, 4)  that passes through  (1, 13) .


the radius of the circle  =  the distance between the center and a point on the circle

the radius of the circle  =  the distance between  (1, 4)  and  (1, 13)


Notice how the (1, 4)  and  (1, 13)  both have the same  x  coordinate.

So the distance between the two points is just  13 - 4 .


the radius of the circle  =  13 - 4   =   9


So we know that  r = 9  ,  h = 1 ,  and  k = 4 .


Plugging these values directly into the standard form of a circle.....


(x - h)2 + (y - k)2  =  r2


(x - 1)2 + (y - 4)2  =  92


(x - 1)2 + (y - 4)2  =  81        This is the equation of the circle, and here's a graph of it.

hectictar  Nov 26, 2017

Thank you what I was looking for was to find the error in my checking of the equation you just wrote. Though I have now figured that out but thank you for your resonse. A stands for the amount dilated horzintoally and b stand for the amount dilated vertically. 

Can someone please close this forum/check that it has been answer?

Guest Nov 26, 2017

15 Online Users

New Privacy Policy

We use cookies to personalise content and advertisements and to analyse access to our website. Furthermore, our partners for online advertising receive information about your use of our website.
For more information: our cookie policy and privacy policy.