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Conic Sections — Class 11 CBSE Mathematics revision notes

A 4-page visual revision sheet for Conic Sections, written for Class 11 CBSE Mathematics. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Conic Sections in one look

circle, parabola, ellipse, hyperbola, one number apart

Conic sections, and where they come from

  • each curve is points P obeying one distance rule: a fixed focus, a fixed directrix
  • eccentricity e=PF/PM, focus distance over directrix distance, is the SINGLE deciding number
  • e=0: circle. 0<e<1: ellipse. e=1: parabola. e>1: hyperbola, and these four ranges never overlap

Eccentricity: the limiting cases

  • e=0 — circle, foci merge at the centre
  • 0<e<1 — ellipse, bounded closed curve
  • e=1 — parabola, one focus, one directrix, one branch
  • e>1 — hyperbola, two unbounded branches

Circle: centre-radius and general form

  • centre (h,k), radius r (x-h)^2+(y-k)^2=r^2
  • general form, centre and radius drop out of g,f,c x^2+y^2+2gx+2fy+c=0
  • centre (-g,-f)
  • radius, real only when g^2+f^2-c>0 r=√g^2+f^2-c

Worked: centre and radius from the general equation

Q. find the centre and radius of the circle x^2+y^2-4x+6y-3=0

  1. match against x^2+y^2+2gx+2fy+c=0: 2g=-4 so g=-2, 2f=6 so f=3, c=-3
  2. centre = (-g,-f)=(2,-3)
  3. radius: r=√g^2+f^2-c=√(-2)^2+3^2-(-3)
  4. =√4+9+3=√16=4

Answer: centre (2,-3), radius 4

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