What does the equation (x – a)2 + (y – b)2 = r2 become when the axes are transferred to parallel axes through the point (a-c, b)?

Given, equation (x – a2) + (y – b)2 = r2. For curious readers- this equation represents a circle in the space centered at point (a, b) having a radius of r units.


To find: Transformed equation of given equation when the coordinate axes are transformed parallelly at point (a - c, b).


We know that, when we transform origin from (0, 0) to an arbitrary point (p, q), the new coordinates for the point (x, y) becomes (x + p, y + q), and hence an equation with two variables x and y must be transformed accordingly replacing x with x + p, and y with y + q in original equation.


Since, origin has been shifted from (0, 0) to (a – c, b); therefore any arbitrary point (x, y) will also be converted as (x + (a – c), y + b) or (x + a - c, y + b).


The given equation (x – a)2 + (y – b)2 = r2 will hence be transformed into the new equation by changing x by x – a + c and y by y – b, i.e. substitution of x by x + a and y by y + b.


= ((x + a - c) – a)2 + ((y – b ) - b)2 = r2


= (x – c)2 + y2 = r2


= x2 + c2 – 2cx + y2 = r2


= x2 + y2 = r2 - c2 + 2cx


Hence, the transformed equation is x2 + y2 = r2 - c2 + 2cx.


1