94 ANALYTIC GEOMETRY
4.3.2-4. Angle between two straight lines.
We consider two straight lines given by the equations
y = k
1
x + b
1
and y = k
2
x + b
2
,(4.3.2.5)
where k
1
=tanϕ
1
and k
2
=tanϕ
2
are the slopes of the respective lines (see Fig. 4.17). The
angle α between these lines can be obtained by the formula
tan α =
k
2
– k
1
1 + k
1
k
2

α
φ
Y
O
Figure 4.17. Angle between two straight lines.
The angle α between the two straight lines given by the general equations
A
1
x + B
1
y + C
1
= 0 and A
2
x + B
2
y + C
2
= 0 (4.3.2.8)
can be calculated using the expression
tan α =
A
1
B
2
– A
2
B
1
A

the sign of the tangent of the angle.
4.3. STRAIGHT LINES AND POINTS ON PLANE 95
4.3.2-5. Point of intersection of straight lines.
Suppose that two straight lines are defined by general equations in the form (4.3.2.8).
Consider the system of two first-order algebraic equations (4.3.2.8):
A
1
x + B
1
y + C
1
= 0,
A
2
x + B
2
y + C
2
= 0.
(4.3.2.10)
Each common solution of equations (4.3.2.10) determines a common point of the tow lines.
If the determinant of system (4.3.2.10) is not zero, i.e.,



A
1
B
1
A

C
1
A
1
B
2
– A
2
B
1
, y
0
=
C
1
A
2
– C
2
A
1
A
1
B
2
– A
2
B
1
.(4.3.2.12)

2
,(4.3.2.15)
and one also says that the slopes of perpendicular straight lines are inversely proportional
in absolute value and opposite in sign.
If the straight lines are given by general equations (4.3.2.8), then a necessary and
sufficient condition for them to be perpendicular can be written as (see Paragraph 4.3.2-4)
A
1
A
2
+ B
1
B
2
= 0.(4.3.2.16)
Example 3. The lines 3x+y–3 =0 and x–3y+8 = 0 are perpendicularsince theysatisfycondition(4.3.2.16):
A
1
A
2
+ B
1
B
2
= 3 ⋅ 1 + 1 ⋅ (–3)=0.
96 ANALYTIC GEOMETRY
4.3.2-7. Condition for straight lines to be parallel.
For two straight lines defined by slope-intercept equations (4.3.2.5) to be parallel and not
to coincide, it is necessary and sufficient that
k

2
φφ
Y
O
Figure 4.18. Parallel straight lines.
Example 4. The straight lines 3x + 4y + 5 = 0 and 3/2 x + 2y + 6 = 0 are parallel since the following
condition (4.3.2.18) is satisfied:
3
3/2
=
4
2
≠
5
6
.
4.3.2-8. Condition for straight lines to coincide.
For two straight lines given by slope-intercept equations (4.3.2.5) to coincide, it is necessary
and sufficient that
k
1
= k
2
, b
1
= b
2
.(4.3.2.19)
If the straight lines are given by general equations (4.3.2.8), then a necessary and
sufficient condition for them to coincide has the form

2
= 0 (4.3.2.21)
can be found using the formula (see Paragraph 4.3.2-3)
d =
|C
1
– C
2
|

A
2
1
+ B
2
1
.(4.3.2.22)
4.4. SECOND-ORDER CURVES 97
4.3.2-10. Condition for a straight line to separate points of plane.
Suppose that a straight line in the Cartesian coordinate system OXY is given by an equation
of the form
Ax + By + C = 0.(4.3.2.23)
Obviously, this straight line divides the plane into two half-planes. We consider two arbitrary
points A
1
(x
1
, y
1
)andA

2
+ C
have opposite signs.
4.4. Second-Order Curves
4.4.1. Circle
4.4.1-1. Definition and canonical equation of circle.
A curve on the plane is called a circle if there exists a rectangular Cartesian coordinate
system OXY in which the equation of this curve has the form (see Fig. 4.19a)
x
2
+ y
2
= a
2
,(4.4.1.1)
where the point O(0, 0) is the center of the circle and a > 0 is its radius. Equation (4.4.1.1)
is called the canonical equation of a circle.
OO
M
N
XX
a
aa
a
YY
()a ()b
Figure 4.19. Circle.
The circle defined by equation (4.4.1.1) is the locus of points equidistant (lying at the
distance a) from its center. If a circle of radius a is centered at a point C(x
0

2
2
+ x
0

a
2
– x
2
0
+ a
2
arcsin
x
0
a
.(4.4.1.3)
98 ANALYTIC GEOMETRY
4.4.1-2. Parametric and other equations of circle.
The parametric equations of the circle (4.4.1.1) have the form
x = a cos θ, y = a sin θ,(4.4.1.4)
where the angle in the polar coordinate system plays the role of the variable parameter (see
Paragraphs 4.2.1-4 and 4.2.1-5).
The equation of the circle (4.4.1.1) in the polar coordinate system has the form
ρ = a (4.4.1.5)
and does not contain the polar angle θ.
Remark. The form of the equation of a circle in a polar coordinate system depends on the choice of the
pole and the polar axis (see Example 7 in Subsection 4.2.1).
4.4.2. Ellipse
4.4.2-1. Definition and canonical equation of ellipse.

1
B
2
joining the opposite vertices of an ellipse, as well as
their lengths 2a and 2b, are called the major and minor axes, respectively, of the ellipse.
The axes of an ellipse are its axes of symmetry. In Fig. 4.20a, the axes of symmetry of the
ellipse coincide with the axes of the rectangular Cartesian coordinate system OXY .The
numbers a and b are called the semimajor and semiminor axes of the ellipse. The number
c =
√
a
2
– b
2
is called the linear eccentricity, and the number 2c is called the focal distance.
The number e = c/a =

1 – a
2
/b
2
, where, obviously, 0 ≤ e < 1, is called the eccentricity
or the numerical eccentricity. The number p = b
2
/a is called the focal parameter or simply
the parameter of the ellipse.
4.4. SECOND-ORDER CURVES 99
The point O(0, 0) is called the center of the ellipse. The points of intersection A
1
(–a, 0),

M
0
| and r
2
= |F
2
M
0
|, respectively.
Remark. For a = b (c = 0), equation (4.4.2.1) becomes x
2
+ y
2
= a
2
and determines a circle; hence a circle
can be considered as an ellipse for which b = a, c = 0, e = 0,andρ = 0, i.e., the semiaxes are equal to each
other, the foci coincide with the center, the eccentricity is zero (the directrices are not defined), and the focal
parameter is zero.
The area of the figure bounded by the ellipse is given by the formula S = πab. The length
of the ellipse can be calculated approximately by the formula L ≈ π

1.5 (a+b)–
√
ab

.The
area of the figure bounded by the ellipse and the chord passing through the points M(x
0
, y

4.4.2-2. Focal property of ellipse.
The ellipse defined by equation (4.4.2.1) is the locus of points on the plane for which the
sum of distances to the foci F
1
and F
2
is equal to 2a (see Fig. 4.21). We write this property
as
r
1
+ r
2
= 2a,(4.4.2.3)
where r
1
and r
2
satisfy the relations
r
1
=

(x + c)
2
+ y
2
= a + ex,
r
2
=

4.4.2-3. Focus-directrix property of ellipse.
The ellipse determined by equation (4.4.2.1) on the plane is the locus of points for which
the ratio of distances to a focus and the like directrix is equal to e:
r
1



x +
a
e



–1
= e, r
2



x –
a
e



–1
= e.(4.4.2.5)
4.4.2-4. Equation of tangent and optical property of ellipse.
The tangent to the ellipse (4.4.2.1) at an arbitrary point M

(x
0
, y
0
) are given by the formulas (see Paragraph 4.3.2-4)
d
1
=
1
Na
|x
0
e + a| =
r
1
(M
0
)
Na
,
d
2
=
1
Na
|x
0
e – a| =
r
2

0
) are the lengths of the focal radii of M
0
.
The tangent at an arbitrary point M
0
(x
0
, y
0
) of an ellipse forms acute angles ϕ
1
and ϕ
2
with the focal radii of the point of tangency, and
sin ϕ
1
=
d
1
r
1
=
1
Na
,sinϕ
2
=
d
2

condition: k
1
k
2
=–1. Thus any two conjugate diameters of a circle are perpendicular to each other, and each
of the diameters is a principal diameter.