Straight line formulae will help us to solve different types of problems on straight line in co-ordinate geometry.

**1.** If a straight line makes an angle α with the positive direction of the x-axis then the slope or gradient of the line i.e. m = tan α.

**2.** Slope of the line joining the points (x\(_{1}\), y\(_{1}\)) and (x\(_{2}\), y\(_{2}\)) is

m = \(\frac{y_{2} - y_{1}}{x_{2} - x_{1}}\) = \(\frac{\textrm{Difference of ordinates of the given point}}{\textrm {Difference of abscissa of the given point}}\)

**3.** Condition of collinearity of three points (x\(_{1}\), y\(_{1}\)), (x\(_{2}\), y\(_{2}\)) and (x\(_{3}\), y\(_{3}\)) is x\(_{1}\) (y\(_{2}\) - y\(_{3}\)) + x\(_{2}\) (y\(_{3}\) - y\(_{1}\)) + x\(_{3}\) (y\(_{1}\) - y\(_{2}\)) = 0.

**4.** The equation of x-axis is y = 0.

**5.** The equation of y-axis is x = 0.

**6.** The
equation of the line parallel to x-axis at a distance h units from x-axis is, y
= h.

**7.** The
equation of the line parallel to y-axis at a distance k units from y-axis is, x
= k.

**8.** The
equation of a straight line in slope-intercept form is y = mx + b, where m is the slope
of the line and b is the y-intercept.

**9. **The
equation of a straight line in point-slope
form is y - y\(_{1}\) = m (x - x\(_{1}\)) where m is the slope of the line and (x\(_{1}\),
y\(_{1}\)) is a given point on the line.

**10.**The equation of a straight line in
symmetrical form is

\(\frac{\mathrm{x - x_{1}}}{\textrm{cos} \mathrm{\theta}}\) = \(\frac{\mathrm{y - y_{1}}}{\textrm{sin} \mathrm{\theta}}\) = r

Where θ is the inclination of the line, (x\(_{1}\), y\(_{1}\)) is a given point on the line and r is the distance between the points (x, y) and (x\(_{1}\), y\(_{1}\)).

**11. **The equation of a straight line in distance form is

\(\frac{\mathrm{x - x_{1}}}{\textrm{cos} \mathrm{\theta}}\) = \(\frac{\mathrm{y - y_{1}}}{\textrm{sin} \mathrm{\theta}}\) = r

Where θ is the inclination of the line, (x\(_{1}\), y\(_{1}\)) is a given point on the line and r is the distance of the point (x, y) on the line from the point (x\(_{1}\), y\(_{1}\)).

**12.** The
equation of a straight line in two-point form is

\(\frac{y - y_{1}}{x - x_{1}}\) = \(\frac{y_{1} - y_{2}}{x_{1} - x_{2}}\) or, y - y\(_{1}\) = \(\frac{y_{2} - y_{1}}{x_{2} - x_{1}}\) (x - x\(_{1}\))

Where (x\(_{1}\), y\(_{1}\)) and (x\(_{2}\), y\(_{2}\)) are two given points on the line.

**13.** The
equation of a straight line in intercept form is \(\frac{x}{a}\)
+ \(\frac{y}{b}\) = 1

Where a is the x-intercept and b is the y-intercept of the line. The straight line intersects the x-axis at (a, 0) and y-axis at (0, b).

**14.** The
equation of a straight line in normal form is x cos α + y sin α = p where p
(> 0) is the perpendicular distance of the line from the origin and a (0 ≤ α
≤ 2π) is the angle that the drawn perpendicular on the line makes with the
positive direction of the x-axis.

**15.** The
equation of a straight line in general form is ax + by + c = 0 where a, b and c are real
constants (a and b both are not zero).

**16.** To
find the co-ordinates of the point of intersection of two given lines we solve
the equations; the value of x is the abscissa and that of y is the ordinate of
the point of intersection.

**17.** The
equation of any straight line through the point of intersection of the lines a\(_{1}\)x
+ b\(_{1}\)y + c\(_{1}\) = 0
and a\(_{2}\)x + b\(_{2}\)y + c\(_{2}\) = 0 is

a\(_{1}\)x + b\(_{1}\)y + c\(_{1}\) + λ (a\(_{2}\)x + b\(_{2}\)y + c\(_{2}\)) = 0, where λ(≠ 0 or ∞) is an arbitrary constant.

**18.** Three given straight lines are concurrent if the point of intersection of any two of them satisfies the equation of the third straight line.

**19.** If θ be the acute angle between the straight lines y = m\(_{1}\)x + c\(_{1}\) and y = m\(_{2}\)x + c\(_{2}\) then,

tan θ = |\(\frac{m_{2} - m_{1}}{1 + m_{1} m_{2}}\)| or, tan θ = ± \(\frac{m_{2} - m_{1}}{1 + m_{1} m_{2}}\)

**20.** If two straight lines are parallel then their slopes would be equal. Thus, the condition of parallelism for the lines y = m\(_{1}\)x+ c\(_{1}\) and y = m\(_{2}\)x + c\(_{2}\) is, m\(_{1}\) = m\(_{2}\).

**21.** The equation of any straight line parallel to the line ax + by + c 0 is ax + by = k where k is an arbitrary constant.

**22.** Two straight lines are perpendicular to each other if the product of ,their slopes = – 1. Thus, the condition of perpendicularity of the lines y = m\(_{1}\)x + c\(_{1}\) and y = m\(_{2}\)x + c\(_{2}\) is m\(_{1}\) m\(_{2}\) = - 1.

**23.** The equation of any straight line perpendicular to the line ax + by + c = 0 is bx - ay = k where k is an arbitrary constant.

**24.** The two equations a\(_{1}\) x + b\(_{1}\) y + c\(_{1}\) = 0 and a\(_{2}\) x + b\(_{2}\)y + c\(_{2}\) = 0 represent the equation of the same straight line when \(\frac{a_{1}}{a_{2}}\) = \(\frac{b_{1}}{b_{2}}\) = \(\frac{c_{1}}{c_{2}}\).

**25.** Let ax + by + c = 0 be a given straight line and P (x\(_{1}\), y\(_{1}\)) and Q (x\(_{2}\), y\(_{2}\)), two given points. The points P and Q are on the same side or opposite sides of the line ax + by + c = 0 according as (ax + by + c) and (ax\(_{1}\) + by\(_{1}\) + c) are of the same or opposite signs.

The origin and the point P (x\(_{1}\), y\(_{1}\)) are on the same side or opposite sides of the straight line ax + by + c = 0 according as c and (ax\(_{1}\) + by\(_{1}\) + c) are of the same or opposite signs.

**26.** Let P (x\(_{1}\), y\(_{1}\)) be a point not lying on the straight line ax + by + c = 0; then the length of the perpendicular drawn from P upon the line is

±\(\frac{a_{1}x + b_{1}y + c}{\sqrt{a^{2} + b^{2}}}\) or,\(\frac{|a_{1}x + b_{1}y + c|}{\sqrt{a^{2} + b^{2}}}\)

**27.** The equations of the bisectors of the angles between the straight lines a\(_{1}\)x + b\(_{1}\)y + c\(_{1}\) = 0 and a\(_{2}\)x + b\(_{2}\)y + c\(_{2}\) = 0 are,

\(\frac{a_{1}x + b_{1}y + c_{1}}{\sqrt{a_{1}^{2} + b_{1}^{2}}}\) = ±\(\frac{a_{2}x + b_{2}y + c_{2}}{\sqrt{a_{2}^{2} + b_{2}^{2}}}\)

If c\(_{1}\) and c\(_{2}\) are of the same signs then the bisector containing the origin is,

\(\frac{a_{1}x + b_{1}y + c_{1}}{\sqrt{a_{1}^{2} + b_{1}^{2}}}\) = +\(\frac{a_{2}x + b_{2}y + c_{2}}{\sqrt{a_{2}^{2} + b_{2}^{2}}}\).

If c\(_{1}\) and c\(_{2}\) are of opposite signs then the bisector containing the origin is,

\(\frac{a_{1}x + b_{1}y + c_{1}}{\sqrt{a_{1}^{2} + b_{1}^{2}}}\) = -\(\frac{a_{2}x + b_{2}y + c_{2}}{\sqrt{a_{2}^{2} + b_{2}^{2}}}\).

**●**** The Straight Line**

**Straight Line****Slope of a Straight Line****Slope of a Line through Two Given Points****Collinearity of Three Points****Equation of a Line Parallel to x-axis****Equation of a Line Parallel to y-axis****Slope-intercept Form****Point-slope Form****Straight line in Two-point Form****Straight Line in Intercept Form****Straight Line in Normal Form****General Form into Slope-intercept Form****General Form into Intercept Form****General Form into Normal Form****Point of Intersection of Two Lines****Concurrency of Three Lines****Angle between Two Straight Lines****Condition of Parallelism of Lines****Equation of a Line Parallel to a Line****Condition of Perpendicularity of Two Lines****Equation of a Line Perpendicular to a Line****Identical Straight Lines****Position of a Point Relative to a Line****Distance of a Point from a Straight Line****Equations of the Bisectors of the Angles between Two Straight Lines****Bisector of the Angle which Contains the Origin****Straight Line Formulae****Problems on Straight Lines****Word Problems on Straight Lines****Problems on Slope and Intercept**

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