Class 11 Maths Chapter 9 Straight Lines – Revision Notes

A straight line can be described in several equivalent ways once its slope is known, as a point-slope equation, a slope-intercept form, or a general linear equation. Chapter 9 of Class 11 Maths lays out these forms alongside the formulas for distance between two points and from a point to a line.

Last Updated: September 23, 2026

Slope

• m = (y2−y1)/(x2−x1) = tanθ.
• Parallel: m1=m2. Perpendicular: m1×m2=−1.

Equation Forms

• Point-slope: y−y1=m(x−x1).
• Slope-intercept: y=mx+c.
• Intercept form: x/a+y/b=1.
• General form: Ax+By+C=0.

Distance

• Between 2 points: d=√[(x2−x1)²+(y2−y1)²].
• Point from line: d=|Ax1+By1+C|/√(A²+B²).

One-Line Summary

Straight lines are analysed via slope (rate of rise), multiple equation forms depending on given information, and distance formulas for points and lines — with parallel/perpendicular conditions based on slope relationships.

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

How many ways can the equation of a straight line be written, and why do we need more than one form?
A line can be expressed in slope-intercept, point-slope, two-point, intercept, and normal form among others. Different forms are convenient depending on what information is given, such as a point and slope, two points, or the intercepts on the axes.

What does the slope of a line tell us in coordinate geometry?
The slope indicates the steepness and direction of a line. A positive slope means the line rises left to right, a negative slope means it falls, a zero slope is horizontal, and an undefined slope indicates a vertical line.

Written by Satish

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