Class 9 · Chapter 3

Coordinate Geometry

Cartesian coordinate system, quadrants, plotting points.

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Tip: open the NCERT PDF for the original chapter text alongside the notes here.

Detailed theory

Two perpendicular number lines — the horizontal

x

-axis and vertical

y

-axis — meet at the origin

O

. Any point

P

in the plane is identified by an ordered pair

(x, y)

, where

x

is its perpendicular distance from the

y

-axis (signed) and

y

is that from the

x

-axis.

Key concepts & formulas

Coordinates of a point:

(x, y)

;

x

is abscissa,

y

is ordinate.

Origin is

(0, 0)

Quadrant I:

(+, +)

; II:

(-, +)

; III:

(-, -)

; IV:

(+, -)

Points on

x

-axis have

y = 0

; on

y

-axis,

x = 0

Distance from origin

= \sqrt{x^2 + y^2}

Reflection across

x

-axis:

(x, y)\to (x, -y)

; across

y

-axis:

(x, y)\to (-x, y)