Week 2: Rectangular Coordinate System & Straight Lines
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2026-09-12T00:00:00.000Z
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Cartesian plane, distance formula, section formula, slopes, parallel & perpendicular lines, and line equation forms. # Mathematics I — Week 2: Rectangular Coordinate System & Straight Lines IITM’s Week 2 moves into the **rectangular coordinate system, slope, parallel/perpendicular lines, and different representation...

Mathematics I — Week 2: Rectangular Coordinate System & Straight Lines
IITM’s Week 2 moves into the rectangular coordinate system, slope, parallel/perpendicular lines, and different representations/equations of a line.
TL;DR
Week 2 is basically about this chain:
The highest-value ideas are:
- coordinates (x,y)
- distance and midpoint
- slope
- slope-intercept form y=mx+c
- point-slope form
- two-point form
- general form Ax+By+C=0
- parallel lines: same slope
- perpendicular lines: slopes multiply to −1
- vertical/horizontal line edge cases
1. The rectangular coordinate system
The standard Cartesian plane uses two perpendicular axes:
- horizontal axis: x-axis
- vertical axis: y-axis
Their intersection is the origin:
A point is represented by an ordered pair:
where:
- x = horizontal position
- y = vertical position
The order matters.
2. Why is it called Cartesian?
"Cartesian" comes from René Descartes, whose Latinized name was Cartesius.
The important mathematical idea is more interesting than the history:
Geometry can be translated into algebra.
A geometric object such as a line becomes an equation.
For example:
is simultaneously:
- an algebraic equation
- a set of ordered pairs
- a geometric straight line
This algebra-geometry bridge is one of the foundational ideas of analytic geometry.
3. Quadrants
The coordinate plane is divided into four regions.
texty ↑ II | I | ----------+----------→ x | III | IV |
Signs:
| Quadrant | x | y |
|---|---|---|
| I | + | + |
| II | − | + |
| III | − | − |
| IV | + | − |
Mnemonic:
textI (+,+) II (-,+) III (-,-) IV (+,-)
4. Distance between two points
Suppose:
and
Then:
This is just the Pythagorean theorem.
Horizontal change:
Vertical change:
So:
5. Example: distance
For:
we have:
Therefore:
This is the familiar 3-4-5 triangle.
6. Midpoint
The midpoint between:
and
is:
You simply average corresponding coordinates.
Example:
Then midpoint:
7. What is slope?
Slope measures how much y changes as x changes.
For two points:
the slope is:
provided:
Another common notation:
or:
8. Intuition behind slope
Slope answers:
If x increases by 1, approximately how much does y change?
Suppose:
Then:
If:
then:
9. Positive slope
If:
then the line rises as we move left to right.
text/ / /
As x increases:
10. Negative slope
If:
the line falls as we move left to right.
text\ \ \
As x increases:
11. Zero slope
A horizontal line has:
Example:
For any two points on it:
so:
12. Undefined slope
A vertical line has undefined slope.
Example:
All points have the same x coordinate:
so:
which is undefined.
Very common trap:
Vertical slope is not infinity in ordinary elementary algebra.
It is undefined.
13. Equation of a line
The most familiar form is:
where:
- m = slope
- c = y-intercept
The y-intercept occurs where:
Then:
so the line crosses the y-axis at:
14. Example
y=2x+3Here:
and:
So:
- slope =2
- y-intercept =(0,3)
15. Point-slope form
Suppose a line has slope m and passes through:
Then:
This is one of the most useful line formulas.
16. Example: point + slope
Find the line through:
with slope:
Use:
Therefore:
Expand:
so:
17. Two-point form
Suppose the line passes through:
and:
First find slope:
Then substitute into point-slope form:
This is sometimes called the two-point form.
18. Example: line through two points
Find the equation through:
and:
First:
Use point-slope form:
Expand:
Thus:
19. General form of a line
Another common representation is:
where A and B are not both zero.
Example:
To convert into slope-intercept form:
Therefore:
20. Slope from general form
For:
if:
then:
thus:
Therefore:
21. The intercepts of a line
For:
x-intercept
Set:
Then:
so:
assuming A=0.
y-intercept
Set:
Then:
so:
assuming B=0.
22. Intercept form
If a line intersects the axes at:
and:
its equation can be written:
This is called intercept form.
23. Deriving intercept form
The slope between:
and:
is:
Using:
and c=b:
Divide through by b:
Therefore:
24. Parallel lines
Two nonvertical lines are parallel when they have equal slopes:
Example:
and:
Both have slope:
Therefore they are parallel.
Their intercepts differ, so they are distinct lines.
25. Coincident lines
If two equations represent the same line, they are coincident.
Example:
and:
The first equation is simply twice the second.
Therefore these are not two separate parallel lines.
They are the same line.
26. Perpendicular lines
For two nonvertical/nonhorizontal lines:
Equivalently:
This is called the negative reciprocal relationship.
27. Example
Suppose:
Then the slope of a perpendicular line is:
because:
28. Important perpendicular edge case
A horizontal line:
has slope:
A vertical line:
has undefined slope.
They are perpendicular.
So the formula:
cannot literally be used here because one slope is undefined.
Geometric reasoning handles this case.
29. Parallel condition in general form
Suppose:
and:
Their slopes are:
and:
For parallel lines:
Therefore:
assuming the normal nonvertical situation.
30. Perpendicular condition in general form
Using:
we get:
Thus:
so:
This formula is useful because it avoids explicitly computing slopes.
31. Angle of inclination
The angle a line makes with the positive x-axis is called its angle of inclination, usually θ.
Its slope satisfies:
where:
for ordinary line orientation.
32. Examples of slope-angle relation
If:
then:
Horizontal line.
If:
then:
If:
then:
A vertical line corresponds to:
where:
is undefined.
Exactly consistent with vertical slope being undefined.
33. Angle between two lines
For slopes m1 and m2, the angle θ between the lines satisfies:
If:
then:
which means the lines are perpendicular.
34. Collinearity
Three points are collinear if they lie on the same straight line.
For:
one method is to compare slopes:
provided denominators are valid.
35. Safer collinearity test
Slope formulas can become awkward for vertical lines.
A safer determinant-style condition is:
If this equals zero, the three points are collinear.
36. Why that test works
Three points form a triangle.
If they are collinear, the triangle has area:
The area formula is:
So collinearity means:
37. Equation of horizontal and vertical lines
These are worth treating separately.
Horizontal
Through:
the horizontal line is:
Example:
Through:
the horizontal line is:
Vertical
Through:
the vertical line is:
Example:
Through:
the vertical line is:
38. A line as a set of solutions
This is an important conceptual upgrade.
Consider:
The line is not merely "the drawing."
It is the entire set:
Every point satisfying the equation belongs to the line.
Examples:
because:
and:
because:
But:
does not belong because:
This way of thinking becomes useful throughout mathematics.
39. Normal vector intuition
For:
the vector:
is perpendicular to the line.
It is called a normal vector.
This explains why:
appears in some line relationships: it is related to the dot product of normal vectors.
You do not need to over-focus on this if IITM has not yet introduced vectors, but the intuition is useful.
40. Distance from a point to a line
For point:
and line:
the perpendicular distance is:
Example:
Point:
Line:
Then:
41. Intersection of two lines
Consider:
and:
At their intersection, both equations have the same x and y.
Therefore:
so:
Then:
Intersection:
Geometrically, solving simultaneous linear equations means:
Find the common point of the lines.
42. Three possibilities for two lines
For two lines in the plane:
One intersection
Different slopes:
One unique solution.
No intersection
Parallel distinct lines:
but different intercepts.
No solution.
Infinitely many intersections
Same line.
Infinitely many solutions.
This connects geometry directly to systems of equations.
43. Common question pattern: line through point parallel to another
Given:
Find a parallel line through:
Original line:
So:
Parallel line also has:
Use point-slope:
Done.
44. Faster general-form method
If a line parallel to:
is required, keep the same A and B:
Then determine K using the given point.
Example:
Parallel to:
through:
write:
Substitute:
Therefore:
Much faster.
45. Line through a point perpendicular to another
Suppose original slope:
Then perpendicular slope:
If the desired line passes through:
then:
Common mistake: reciprocal without negative sign
If:
the perpendicular slope is:
not:
and not:
Rule:
Common mistake: line slope from general equation
Given:
Do not say:
without checking signs.
Rearrange:
Thus:
Here it happens to be positive.
General shortcut:
For:
we get:
Common mistake: confusing intercepts
For:
5 is the y-intercept, not the x-intercept.
To find the x-intercept:
so:
Thus x-intercept:
Common mistake: treating ordered pairs as unordered
For coordinates:
Unlike sets:
The entire coordinate system depends on order.
This connects directly to Week 1's Cartesian products, where ordered pairs also appeared.
50. Week 1 → Week 2 connection
Week 1 introduced:
and ordered pairs:
Week 2 specializes this to:
The coordinate plane is essentially:
Every point is an ordered pair of real numbers.
So the syllabus is more connected than it initially looks.
51. One very useful mental model
A straight line equation is a constraint on two variables.
For:
x and y cannot vary independently.
Once x is chosen:
is forced.
For example:
This is also why a nonvertical line can be interpreted as a function:
52. Vertical line and function connection
Consider:
At:
there are infinitely many y values.
Therefore it cannot be written as:
as an ordinary real-valued function of x.
This foreshadows the vertical-line test that appears later in the course.
53. Formula sheet
Distance
d=(x2−x1)2+(y2−y1)2Midpoint
M=(2x1+x2,2y1+y2)Slope
m=x2−x1y2−y1Slope-intercept form
y=mx+cPoint-slope form
y−y1=m(x−x1)General form
Ax+By+C=0Slope from general form
m=−BAParallel
m1=m2Perpendicular
m1m2=−1Point-to-line distance
d=A2+B2∣Ax0+By0+C∣54. What you should actually master
Don't judge Week 2 by whether you have read all of this.
You should be able to do these without much hesitation:
- Plot/read (x,y) coordinates.
- Identify quadrants.
- Find distance between two points.
- Find midpoint.
- Calculate slope.
- Recognize positive, negative, zero and undefined slopes.
- Construct a line from a point and slope.
- Construct a line from two points.
- Move between: y=mx+c and Ax+By+C=0.
- Find x- and y-intercepts.
- Test whether lines are parallel.
- Test whether lines are perpendicular.
- Construct parallel/perpendicular lines through a given point.
- Determine whether points are collinear.
- Solve for the intersection of two lines.
If those are comfortable, Week 2 is basically done.
55. Week 2 conceptual map
textCoordinate plane │ ├── point (x,y) ├── distance └── midpoint │ ▼ Slope │ ├── positive ├── negative ├── zero └── undefined │ ▼ Straight line │ ├── y = mx + c ├── point-slope ├── two-point ├── intercept form └── Ax + By + C = 0 │ ▼ Relationships │ ├── parallel ├── perpendicular ├── coincident └── intersecting
What comes next
Week 3 = Quadratic Functions: quadratic functions, their graphs, minima/maxima, vertex, slope-related interpretation, and quadratic equations.
That is where Maths 1 starts becoming more function-centric rather than purely coordinate geometry.
Quick Recall & Flashcards
Test your active recall before proceeding to practice problems. Format:
Question :: Answer.- Distance between (x1,y1) and (x2,y2) :: (x2−x1)2+(y2−y1)2
- Slope of line through (x1,y1) and (x2,y2) :: m=x2−x1y2−y1
- Condition for parallel lines with slopes m1,m2 :: m1=m2
- Condition for perpendicular lines with slopes m1,m2 :: m1m2=−1⟹m2=−m11
- Slope of general line ax+by+c=0 :: m=−ba
- Perpendicular distance from (x0,y0) to ax+by+c=0 :: a2+b2∣ax0+by0+c∣
- Midpoint of segment between (x1,y1) and (x2,y2) :: (2x1+x2,2y1+y2)
Practice & Self-Check
Attempt these diagnostic questions to verify your operational understanding.
1
Question 1
What is the slope of a line that is perpendicular to the line 3x−6y+7=0?
MCQ
3 Unit Assessment
A. 1/2
B. -2
C. 2
D. -1/2
2
Question 2
What is the perpendicular distance from the point (1,2) to the line 3x+4y−1=0?
NAT
3 Unit Assessment