Question 29·Medium·Circles
In the coordinate plane shown, circle has center and passes through point .
Which choice is an equation of circle ?
For equation-of-a-circle questions, first read the center from the figure and immediately write the left side as . Then compute using squared horizontal and vertical changes to a point on the circle; using avoids unnecessary square roots and helps prevent arithmetic errors.
Hints
Identify key points
Use the graph to read the coordinates of the center and the point on the circle.
Find the radius squared
Compute from to to get (you do not need to take a square root).
Use standard form
Plug the center and into .
Desmos Guide
Plot the given points
Enter the points and so you can refer to them on the graph.
Test each equation by graphing
Enter each answer choice as an equation (one at a time or all together). Look for the circle whose center matches and that passes through .
Confirm with the radius
For the circle that fits, use the distance from to (visually or by calculation) to check that its radius squared matches the constant on the right side.
Step-by-step Explanation
Read the center and a point on the circle
From the graph, the center is and the circle passes through . So the radius is the distance from to .
Compute using the distance formula idea
Compute the horizontal and vertical changes from to :
and .
So
Write the circle equation in standard form
A circle with center and radius has equation .
Here and , so the equation is .