Question 43·Medium·Circles
A cell phone tower’s coverage boundary on a map is modeled by the equation
Which choice gives the equation of this circle in the form ?
When you see and with the same coefficient, aim to rewrite in . Group terms and terms, move the constant, then complete the square for each group. Be careful: means the center’s -coordinate is , and the right side after balancing is .
Hints
Recognize the circle pattern
If an equation has and (same coefficient) and no term, it can be rewritten as a circle by completing the square.
Group and move the constant
Put the constant on the right side, then group the terms together and the terms together.
Complete the square twice
For , add and subtract . For , add and subtract .
Desmos Guide
Graph the original equation
In Desmos, enter to graph the circle.
Graph an answer choice
Enter one answer choice, such as , and see whether it matches the original circle exactly.
Confirm by matching graphs
The correct choice is the one whose graph lies exactly on top of the graph from the original equation.
Step-by-step Explanation
Group -terms and -terms
Start with
Move the constant term to the other side and group:
Complete the square
Complete the square for each variable:
Substitute:
Write the standard form
Add and to both sides:
So the circle’s equation is .