# Equation of a Straight Line

- The angle which a line makes with the positive direction of x-axis measured in the anti-clockwise direction is called the inclination of the line.
- If ( 90°) is the inclination of a line, then tan is called its slope (or gradient).
- Slope of a line.

(i) If the inclination of a line is , its slope = m = tan .

(ii) Slope of a line through (x_{1}, y_{1}) and (x_{2}, y_{2}) is given by m =(y_{2}-y_{1)/(}x_{2}-x_{1)} - Equation of a straight line.

(i) Equation of a line parallel to x-axis is y = b.

(ii) Equation of a line parallel to y-axis is x = a.

(iii) Equation of a line with slope m and y-intercept c is y = m x +c.

(iv) Equation of a line through (x_{1}, y_{1}) and with slope m is y -y_{1}= m (x -x_{1}). - Conditions of parallelism and perpendicularity.

Two lines with slopes m_{1}and m_{2}are

(i) parallel if and only if m_{1}= m_{2}.

(ii) perpendicular if and only if m_{1}m_{2}= -1. - Reflection of P(, ) in the line y = x is P'(,).

## Exercise

- Find the inclination of a line whose gradient is

(i) 1 (ii) 3 (iii) 1/3 - Find the equation of a straight line parallel to x-axis and passing through the point (2, -7).
- Find the equation of a straight line whose

(i) gradient = 3, y-intercept = -4/3

(ii) inclination = 30°, y-intercept = -3. - Write down the gradient and the intercept on the y-axis of the line 3y +2x = 12.
- The equation to the line PQ is 3y -3x +7 = 0.

(i) Write down the slope of the line PQ.

(ii) Calculate the angle that the line PQ makes with the positive direction of x-axis. - The given figure represents the lines y = x +1 and y =
3 x -1. Write down the angles which
the lines make with the positive direction of x-axis. Hence determine
.

[**Hint.**Ext. = sum of two opposite int. s; 60° = +45°.]

- Given that (a, 2a) lies on the line y/2 = 3x -6, find the value of a.
- The graph of the equation y = mx +c passes through the points (1, 4) and (-2, -5). Determine the values of m and c.
- Find the equation of a straight line passing through (-1, 2) and having y-intercept 4 units.
- Find the equation of a st. line whose inclination is 60° and passes through the point (0, -3).
- Given that the line y/2 = x -p and the line ax +5=3y are parallel, find the value of a.
- Find the value of m, if the lines represented by 2mx -3y = 1 and y = 1 -2x are perpendicular to each other.
- If the lines 3x +y = 4, x -ay +7 = 0 and bx +2y +5 = 0 form three consecutive sides of a rectangle, find the values of a and b.
- Find the equation of a straight line perpendicular to the line 2x +5y +7 = 0 and with y-intercept -3 units.
- Find the equation of a straight line parallel to the line 2x +3y = 5 and having the same y-intercept as x +y +4 = 0.
- Find the equation of the line which is parallel to 3x -2y -4 = 0 and passes through the point (0, 3).
- Write down the equation of the line perpendicular to 3x +8y = 12 and passing through the point (-1, -2).
- The co-ordinates of two points E and F are (0, 4) and (3, 7) respectively. Find

(i) the gradient of EF

(ii) the equation of EF

(iii) the co-ordinates of the point where the line EF intersects the x-axis. - Find the equation of the line passing through the points (4, 0) and (0, 3). Find the value of k, if the line passes through (k,3/2).
- If A(-1, 2), B(2, 1) and C(0, 4) are the vertices of a ABC, find the equation of the median through A.
- Find the equation of a line passing through the point (-2, 3) and having x-intercept 4 units.

[**Hint.**Since x-intercept is 4, the line passes through (4, 0).] - Find the equation of the st. line containing the point (3, 2) and making positive equal intercepts on the axes.
- The intercepts made by a st. line on the axes are -3 and 2 units. Find

(i) the gradient of the line.

(ii) the equation of the line.

(iii) the area of the triangle enclosed between the line and the co-ordinate axes. - A line through the point P (2, 3) meets the co-ordinate axes at points A
and B. If PA = 2 PB, find the co-ordinates of A and B. Also find the equation of the line AB.

- Calculate the co-ordinates of the point of intersection of the lines represented by x +y = 6 and 3x -y = 2.
- The line joining the points P (4, k) and Q (-3, -4) meets the x-axis at A
(1, 0) and y-axis at B. Find

(i) the value of k.

(ii) the ratio of PB : BQ. - Find the equations of the diagonals of a rectangle whose sides are x = -1, x = 2, y = -2 and y = 6.
- Find the co-ordinates of the image of (3, 1) under reflection in x-axis followed by reflection in the line x =1.
- If P'(-4, -3) is the image of the point P under reflection in the origin, find

(i) the co-ordinates of P.

(ii) the co-ordinates of the image of P under reflection in the line y = -2. - Find the co-ordinates of the image of the point P(4, 3) under reflection in the x-axis followed by reflection in the line x = -2.

## Answers

**1.**(i) 45° (ii) 60° (iii) 30°.

**2.**y +7 = 0

**3.**(i) 33x -3y -4 = 0 (ii) x -3y -33 = 0

**4.**-2/3; 4

**5.**(i) 1 (ii) 45°

**6.**45°, 60°; 15°

**7.**3

**8.**m = 3, c = 1

**9.**y = 2x +4

**10.**3x -y -3 = 0

**11.**6

**12.**3/4

**13.**a = 3, b = 6

**14.**5x -2y -6 = 0

**15.**2x +3y +12 = 0

**16.**3x -2y +6 = 0

**17.**8x -3y +2 = 0

**18.**(i) 1 (ii) x -y +4 = 0 (iii) (-4, 0)

**19.**3x +4y -12 = 0; 2

**20.**x -4y +9 = 0

**21.**x +2y -4 = 0

**22.**x +y -5 = 0

**23.**(i) 2/3 (ii) 2x -3y +6 = 0 (iii) 3 sq. units

**24.**A (6, 0), B (0, 9/2); 3x +4y -18 = 0

**25.**(2, 4)

**26.**(i) 3 (ii) 4 : 3

**27.**8x-3y+2 = 0, 8x+3y-10 = 0

**28.**(-1, -1)

**29.**(i) (4, 3) (ii) (4, -7)

**30.**(-8, -3)