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Circles for CAT: Formulas, Weightage & Question Types

Curved areas, chords, angles in a circle, tangents and touching circles.

Circles formula sheet

Everything CAT needs from Circles, in one place. Bookmark it for revision; the lesson in the app explains where each line comes from and when to reach for it.

Everything for this chapter, in one place. rr is the radius, θ\theta an angle at the centre in degrees.

Basics

Circumference2πr2\pi r
Areaπr2\pi r^2
Arc lengthθ360×2πr\dfrac{\theta}{360} \times 2\pi r
Sector areaθ360×πr2\dfrac{\theta}{360} \times \pi r^2
Segment areasector −- triangle
Triangle inside a sector12r2sin⁡θ\dfrac{1}{2}r^2\sin\theta

Chords

The perpendicular from the centrebisects the chord
So, with dd the distance from the centre(chord2)2+d2=r2\left(\dfrac{\text{chord}}{2}\right)^2 + d^2 = r^2
Chord from the central angle2rsin⁡θ22r\sin\dfrac{\theta}{2}
Equal chordsare equidistant from the centre
Intersecting chords at EEAE×EB=CE×EDAE \times EB = CE \times ED

Angles

Angle at the centretwice the angle at the circumference on the same arc
Angle in a semicircle90∘90^\circ
Angles in the same segmentare equal
Cyclic quadrilateralopposite angles add to 180∘180^\circ

Tangents

A tangentis perpendicular to the radius at the point of contact
Two tangents from one pointare equal in length
Length of a tangent from distance ddd2−r2\sqrt{d^2-r^2}
Two tangents meeting at angle 2θ2\thetathe point is at rsin⁡θ\dfrac{r}{\sin\theta} from the centre
With an inner tangent line ABAB∠AOB=90∘−∠APB2\angle AOB = 90^\circ - \dfrac{\angle APB}{2}

Touching circles

Touching externallycentres are r1+r2r_1+r_2 apart
Touching internallycentres are ∣r1−r2∣\lvert r_1-r_2 \rvert apart
Three circles on a common tangent line1r3=1r1+1r2\dfrac{1}{\sqrt{r_3}} = \dfrac{1}{\sqrt{r_1}} + \dfrac{1}{\sqrt{r_2}}
Three equal circles of radius rr, mutually touchingthe enclosing circle has r(1+23)r\left(1+\frac{2}{\sqrt3}\right), the inner one r(23−1)r\left(\frac{2}{\sqrt3}-1\right)

The 5 Circles question types CAT repeats

Past CAT papers return to the same few shapes. Learn to recognise them and most questions become a known routine.

  1. 01

    The area of a curved region

  2. 02

    A chord, and its distance from the centre

  3. 03

    Angles in a circle

  4. 04

    Circles that touch each other

  5. 05

    Tangents, or intersecting chords

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Circles in CAT: quick answers

How many Circles questions come in CAT Quant?
About 0.4 questions in a 22-question Quant section, worth roughly 1 marks. Circles appeared in 13 of the 24 CAT papers from 2017 to 2025, 16 questions in all, which ranks it 21 of 25 Quant chapters.
Are Circles questions in CAT MCQ or TITA?
19% of the Circles questions asked since 2017 were TITA (type in the answer, no negative marking). The rest were multiple choice.
What types of Circles questions does CAT ask?
CAT keeps returning to 5 question shapes in Circles: The area of a curved region; A chord, and its distance from the centre; Angles in a circle; Circles that touch each other; Tangents, or intersecting chords.

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