Q.Find the equation of the circle with centre (0,2) and radius 2.
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Concept understanding — Circle Equation Standard Form
Where the Circle Equation Comes From
Imagine you're standing at a point on a flat field. You tie a rope to a stake at that point, walk out the full length of the rope, and start walking in a circle, keeping the rope taut. Every point you step on is exactly the same distance from the stake.
That's the entire idea: a circle is the set of all points that are a fixed distance (the radius) from a fixed point (the centre).
If we put this on a coordinate plane, we can turn that geometric idea into an algebraic equation.
From Geometry to Algebra
Let the centre be at coordinates (h,k). Let the radius be r. Take any point (x,y) that lies on the circle. The distance from (x,y) to (h,k) must equal r.
What's the distance between two points in the plane? The distance formula:
(x−h)2+(y−k)2=r
Now square both sides to remove the square root:
(x−h)2+(y−k)2=r2
That's it. That's the standard form of the equation of a circle.
(x−h)2+(y−k)2=r2
where (h,k) is the centre and r is the radius (r>0).
What Each Piece Tells You
(x−h) and (y−k) — these shift the circle away from the origin. If the centre is at (0,0), the equation simplifies to x2+y2=r2.
r2 — notice it's the square of the radius, not the radius itself. If the equation says x2+y2=25, the radius is 25=5, not 25.
The equals sign — only the points (x,y) that make this equation true lie on the circle. Any other point gives a larger or smaller left-hand side.
Watch out
A common mistake: for (x−3)2+(y+2)2=16, students often read the centre straight off the signs printed in the equation and say (3,2). That's wrong. Each bracket must first be written in the exact form x−h and y−k: here (y+2)=(y−(−2)), so k=−2, not 2. The centre is actually (3,−2). Always flip the sign inside every bracket before reading off h and k.
Quick Example
Write the equation of a circle with centre (−1,4) and radius 3.
Here h=−1, k=4, r=3. Plug in:
(x−(−1))2+(y−4)2=32
Simplify:
(x+1)2+(y−4)2=9
That's the standard form. From this, you can immediately read off the centre (−1,4) and radius 3.
Why This Form Matters
The standard form is the most useful because it gives you the centre and radius at a glance. In exams, you'll often be given an expanded form like x2+y2−6x+4y−12=0 and asked to rewrite it in standard form by completing the square — that's the next step in your learning, but the standard form itself is the destination.
For now: centre tells you where, radius tells you how big, and the equation tells you which points belong.
The Standard Form of a Circle's Equation is one of the first results in the NCERT Class 11 Mathematics chapter on Conic Sections, matching searches like "equation of a circle: definition, formula and examples" or "conic sections important questions class 11 maths". Recognising centre and radius directly from this form is also a routine, quick-scoring question type in CBSE boards, JEE Main, and state CET coordinate geometry sections.
Concept: Standard form of a circle equation
A circle with centre (h,k) and radius r has the equation (x−h)2+(y−k)2=r2.
Here the centre is (0,2), so h=0 and k=2. The radius is r=2.
Substituting into the standard form:
(x−0)2+(y−2)2=22
x2+(y−2)2=4
✓Final answer
The equation of the circle is x2+(y−2)2=4.
A circle is the set of all points at a fixed distance (radius) from a center; substituting center (0,2) and radius 2 into the standard form gives (x−0)2+(y−2)2=4.
The standard form of a circle's equation captures its geometric definition perfectly: every point (x,y) on the circle is exactly r units away from the center (h,k). The distance formula (x−h)2+(y−k)2=r squares to eliminate the root, giving us the clean algebraic form (x−h)2+(y−k)2=r2.
This form makes sense because the left side measures the squared distance from any point (x,y) to the center, and the right side is the squared radius — so the equation says "all points whose squared distance from the center equals the squared radius," which is precisely what a circle is.
(x−h)2+(y−k)2=r2
where (h,k) is the center and r is the radius.
Now we substitute the given values directly.
Identify the center coordinates: We have h=0 and k=2.
Identify the radius: We have r=2, so r2=4.
Substitute into the standard form:
(x−0)2+(y−2)2=4
Simplify: Since x−0=x, we get
x2+(y−2)2=4
You could expand (y−2)2 to write this as x2+y2−4y+4=4, which simplifies to x2+y2−4y=0, but the form x2+(y−2)2=4 is cleaner and immediately reveals the circle's center and radius.
Tip
When the center has a zero coordinate, the corresponding term simplifies beautifully: (x−0)2=x2. This circle sits on the y-axis, centered at (0,2).