Question

If $$\left( { - 4,\,5} \right)$$  is one vertex and $$7x - y + 8 = 0$$    is one diagonal of a square, then the equation of second diagonal is :

A. $$x + 3y = 21$$
B. $$2x - 3y = 7$$
C. $$x + 7y = 31$$  
D. $$2x + 3y = 21$$
Answer :   $$x + 7y = 31$$
Solution :
One vertex of square is $$\left( { - 4,\,5} \right)$$  and equation of one diagonal is $$7x - y + 8 = 0$$
Diagonal of a square are perpendicular and bisect each other.
Let the equation of the other diagonal be $$y = mx + c$$   where $$m$$ is the slope of the line and $$c$$ is the $$y$$-intercept.
Since this line passes through $$\left( { - 4,\,5} \right)$$
$$\therefore \,5 = - 4m + c......\left( {\text{i}} \right)$$
Since this line is at right angle to the line $$7x - y + 8 = 0$$    or $$y = 7x + 8,$$   having slope $$ = 7,$$
$$\therefore \,7 \times m = - 1{\text{ or }}\,m = \frac{{ - 1}}{7}$$
Putting this value of m in equation $$\left( {\text{i}} \right)$$ we get $$C = \frac{{31}}{7}$$
Hence, equation of the other diagonal is $$y = - \frac{1}{7}x + \frac{{31}}{7}{\text{ or }}x + 7y = 31$$

Releted MCQ Question on
Geometry >> Straight Lines

Releted Question 1

The points $$\left( { - a, - b} \right),\left( {0,\,0} \right),\left( {a,\,b} \right)$$     and $$\left( {{a^2},\,ab} \right)$$  are :

A. Collinear
B. Vertices of a parallelogram
C. Vertices of a rectangle
D. None of these
Releted Question 2

The point (4, 1) undergoes the following three transformations successively.
(i) Reflection about the line $$y =x.$$
(ii) Translation through a distance 2 units along the positive direction of $$x$$-axis.
(iii) Rotation through an angle $$\frac{p}{4}$$ about the origin in the counter clockwise direction.
Then the final position of the point is given by the coordinates.

A. $$\left( {\frac{1}{{\sqrt 2 }},\,\frac{7}{{\sqrt 2 }}} \right)$$
B. $$\left( { - \sqrt 2 ,\,7\sqrt 2 } \right)$$
C. $$\left( { - \frac{1}{{\sqrt 2 }},\,\frac{7}{{\sqrt 2 }}} \right)$$
D. $$\left( {\sqrt 2 ,\,7\sqrt 2 } \right)$$
Releted Question 3

The straight lines $$x + y= 0, \,3x + y-4=0,\,x+ 3y-4=0$$         form a triangle which is-

A. isosceles
B. equilateral
C. right angled
D. none of these
Releted Question 4

If $$P = \left( {1,\,0} \right),\,Q = \left( { - 1,\,0} \right)$$     and $$R = \left( {2,\,0} \right)$$  are three given points, then locus of the point $$S$$ satisfying the relation $$S{Q^2} + S{R^2} = 2S{P^2},$$    is-

A. a straight line parallel to $$x$$-axis
B. a circle passing through the origin
C. a circle with the centre at the origin
D. a straight line parallel to $$y$$-axis

Practice More Releted MCQ Question on
Straight Lines


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