Question 45·Medium·Circles
In the -plane, a circle has center at and passes through the point . Which equation represents this circle?
For circle-equation questions, first recall the standard form , then immediately match the center: pick the choice where the signs inside the parentheses give the correct . Next, use the given point on the circle to compute by substituting its coordinates and simplifying, and reject any option with the wrong . This two-step process—center first, radius second—lets you eliminate wrong answers quickly without extra algebra.
Hints
Use the standard circle equation
Think of the general form of a circle with center and radius : how do and appear in the equation?
Match the center
In the expression , what values of and give you ? Which answer choices place those values correctly (watch the signs inside the parentheses)?
Find the radius using the given point
Once you have the correct center in the equation, plug in the coordinates of the point to find . What value do you get?
Desmos Guide
Graph all answer choices
In Desmos, enter each option on a separate line exactly as written, for example: (x-3)^2+(y+2)^2=32. You should see up to four circles on the graph.
Plot the given points
Add two points: type (3,-2) and (7,2) on separate lines so their locations appear clearly on the graph.
Check center and point on the circle
Look at which circle has its center at the point (3,-2) (it should be exactly in the middle of the circle) and also passes through the point (7,2). The equation corresponding to that circle is the correct choice.
Step-by-step Explanation
Recall the standard form of a circle
The equation of a circle with center and radius in the -plane is
Our job is to match this form using the given center and point on the circle.
Write the equation using the given center
The center is , so and .
Substitute these into the standard form:
which simplifies to
Any correct answer must have and on the left side.
Use the point on the circle to find
The circle passes through , so this point must satisfy the equation.
Substitute and into :
Now calculate each part to find .
Compute and write the final equation
Compute the expression:
So .
Substitute this back into the circle equation:
which is the equation of the circle.