Question 8·Medium·Circles
Consider the circle defined by the equation
Which of the following points lies outside the circle?
For circle questions in standard form , immediately identify the center and radius , and remember that is the squared distance from the center. To check if a point is inside, on, or outside the circle, quickly substitute its coordinates into this expression and compare the result to : less than means inside, equal means on, and greater than means outside. This substitution method is usually faster and less error-prone than trying to estimate visually or convert to another form.
Hints
Use the form of the circle equation
Compare the equation to the standard form . What are the center and radius of this circle?
Relate distance to the equation
For a point , think about what the value of tells you in relation to the circle’s radius squared, which is .
Test each point systematically
Substitute the - and -coordinates of each answer choice into and compare each result to . Which one gives a value greater than ?
Desmos Guide
Graph the circle
In Desmos, enter the equation (x-3)^2 + (y+2)^2 = 49 to graph the circle with center and radius .
Plot each answer choice as a point
On separate lines, enter each point as an ordered pair: (4,-2), (10,-2), (-4,5), and (3,5). Desmos will display each as a point on the graph.
Visually compare positions
Look at where each point is in relation to the circle: identify which point is clearly beyond the circle’s boundary, not on or inside it. That point is the one that lies outside the circle.
(Optional) Verify with the expression value
You can also type the expression (x-3)^2 + (y+2)^2 and use a table with each - and -value to see the numerical result for each point; the point whose value is greater than 49 is the one outside the circle.
Step-by-step Explanation
Identify the center and radius of the circle
The given circle is
A circle in the form has center and radius .
So here:
- , , so the center is .
- , so the radius is .
Translate “outside the circle” into an algebra condition
For any point , the left side of the equation
represents the square of the distance from to the center .
- If , the point is inside the circle.
- If , the point is on the circle.
- If , the point is outside the circle.
We want the point whose value is greater than 49.
Evaluate the circle expression for each answer choice
Compute for each point:
- A) :
which is less than (inside).
- B) :
which equals (on the circle).
- C) :
which is greater than (candidate for outside).
- D) :
which equals (on the circle).
Choose the point that lies outside the circle
Only the point whose value of is greater than 49 lies outside the circle. From the calculations, gives , so the point outside the circle is .