LEARN · EXPLAIN · REVISE

Find the radius of the circle.

Read the idea, work independently, then explain what changed.

高二選擇性必修第一册(A版).pdf · 2.4 · PDF 87 / printed page 82

TOPIC 01

Equations of a circle

Construct circle equations from centers, radii, diameters and point constraints.

Construct circle equations from centers, radii, diameters and point constraints.
Justify the method and check the conditions in a new situation.

Use one hint at a time. A correction explains what changed, not just the final answer.

01 / Foundation#Your turn

(x-2)^2+(y+1)^2=25

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Complete squares before reading the geometric parameters.

Hint 2

Use this intermediate relation.

r^2=t^2

Worked solution

The requested value is 5.

Checks and common pitfalls: The radius is the positive square root.

Think first. Reveal a hint when the class is ready.

Teacher preparation and assessment

Construct circle equations from centers, radii, diameters and point constraints.
Which condition is essential in equations of a circle?
Predict the shape when the constant of a circle equation changes through the zero-radius threshold.

Standard form: A circle is the locus at fixed positive distance from its center.
$(x-h)^2+(y-k)^2=r^2$
Existence: In x²+y²+Dx+Ey+F=0, a nondegenerate circle requires (D²+E²)/4−F>0.

A positive squared radius is necessary; zero gives a single point, and negative gives no real locus.