Circles Flashcards
Diametric form
(x-x1)(x-x2) + (y-y1)(y-y2) = 0
r =
√g^2 + f^2 - C
Power of a pt
S1 = x1^2 + y1^2 + 2gx1 + 2fy1 + C
If S1 > 0
pt lies out
Length of tangent
√S1
Intercept cut off by circle from x axis
2√g^2-C^2
Eqn of line
lx + my + n = 0
If r1+r2 < C1C2
circles don’t touch
If r1+r2>C1C2
Circles touch externally
If r1 + r2 = C1C2
Circles intersect at 2 pts
If r1-r2 = C1C2
Circles touch internally
If r1-r2 < C1C2
One circle in other
Common chord eqn
S1 - S2 = 0
director circle
x^2+y^2 = 2r^2
angle b/n circles cos(180-θ) =
[r1^2 + r2^2 - (C1C2)^2]/2r1r2
If 2 circles cut orthogonally
r1^2 + r2^2 = (C1C2)^2
No of common tangents
4 if they don’t touch
3 if they touch
2 if they intersect at 2 pts
1 if one is in other & touches
0 if one in other
Eqn of tangent to circle
xx1 + yy1 = r^2
Condition for line to be tangent to x^2 + y^2 = r^2
C^2 = r^2(1+m^2)
Parametric eqns
x = rcosθ
y = rsinθ
If e = 0
circle
If e = 1
parabola
if e < 1
ellipse
For y^2 = 4ax
focus(a,0)
directrix x = -a
LR = 4a
focal dist = |x+a|