# Class 11 RD Sharma Solutions- Chapter 22 Brief Review of Cartesian System of Rectangular Coordinates- Exercise 22.1

### Question 1. If the line segment joining the points P(x_{1},y_{1}) and Q(x_{2},y_{2}) subtends an angle ∅ at the origin O, prove that OP.OQ cos∅ = x_{1}x_{2} + y_{1}y_{2}.

**Solution:**

Since, O is the origin, we can get, OP

^{2}= x_{1}^{2}+ y_{1}^{2 }and OQ^{2 }= x_{2}^{2}+ y_{2}^{2}Also, by the distance formula we know distance between two points P and Q is:

PQ

^{2}= (x_{2}– x_{1})^{2}+ (y_{2}– y_{1})^{2}Using the cosine formula, in triangle OPQ, we have

PQ

^{2}= OP^{2}+ OQ^{2}– 2 (OP). (OQ) cos∅⇒ (x

_{2}– x_{1})^{2}+ (y_{2}– y_{1})^{2}= x_{1}^{2}+ y_{1}^{2}+ x_{2}^{2}+ y_{2}^{2}– 2 (OP). (OQ) cos∅⇒ x

_{2}^{2 }+ x_{1}^{2}– 2 x_{2}x_{1}+ y_{2}^{2}+ y_{1}^{2}– 2y_{2}y_{1}= x_{1}^{2}+ y_{1}^{2}+ x_{2}^{2}+ y_{2}^{2}– 2 (OP). (OQ) cos∅⇒ – 2 (x

_{1}x_{2}+ y_{1}y_{2}) = – 2 (OP). (OQ) cos∅⇒ OP.OQ cos∅ = x

_{1}x_{2}+ y_{1}y_{2}

### Question 2. The vertices of a triangle ABC are A(0,0), B (2,-1) and C (9,0). Find cos B.

**Solution:**

Using the cosine formula, we know

Now, lets assume, a = BC, b = CA and c = AB are the sides of the triangle ABC.

Therefore, the distance between two consecutive points can be calculated as:

a = BC =

b = CA =

and c = AB =

Using the cosine formula,

Hence,

### Question. 3 Four points A(6,3), B(-3,5), C(4,-2) and D(x,3x) are given in such a way that , find x.

**Solution:**

We know the formula that,

area of a triangle = , so

area of triangle ABC =

=

=

Similarly, area of triangle DBC =

=

=

Now, we are given that

### Question. 4 The points A(2,0), B(9,1), C(11,6) and D (4,4) are the vertices of a quadrilateral ABCD. Determine whether ABCD is a rhombus or not.

**Solution: **

We know the property of a rhombus that the diagonals bisect each other at the right angles. Thus, both the diagonals must have a common mid-point.

Now mid-point of line AC = = (13/2, 3)

and mid-point of line BD = =

Since, both the lines have different mid-points, we can conclude that the quadrilateral ABCD is not a rhombus.

### Question 5. Find the coordinates of the centre of the circle inscribed in a triangle whose vertices are (-36,7), (20,7), and (0,-8).

**Solution:**

Since, the circle is inscribed in a triangle, the centre of the circle is known as incentre. We know that incentre of a circle (O) inscribed in a triangle is given by the formula:

O = , where a, b, c are length opposite to ∠A, ∠B and ∠C respectively.

Therefore, lets say a = BC =

similarly, b = AC =

and c = AB =

Therefore, the coordinates of the incentre will be:

O =

= (-1,0)

Hence, the coordinate of the centre of the circle is (-1,0)

### Question 6. The base of an equilateral triangle with side 2a lies along the y-axis such that the mid-point of the base is at the origin. Find the vertices of the triangle.

**Solution:**

Since, ABC is an equilateral triangle it will have each sides equal, i.e, AB = BC = CA = 2a

Also, area of an equilateral triangle = , where a is the side of the triangle.

Therefore, area of given triangle =

Also, area of a triangle =

Thus, the coordinates of point A is

similarly, the coordinates of point B is (0,-a) and the coordinates of point C is (0,a)

Hence, the vertices of a triangle are (0,a), (0,-a) and or (0,a), (0.-a) and

### Question 7. Find the distance between P(x1,y1) and Q(x2,y2) when (i) PQ is parallel to the y-axis, (ii) PQ is parallel to the x-axis.

**Solution: **

We are given two points P(x1,y1) and Q(x2,y2),

(i) when line PQ is parallel to the y-axis, then we can conclude that the x-coordinate will be constant ⇒ x2 = x1

Thus, by using distance formula:

PQ =

(ii) when line PQ is parallel to the x-axis, then we can conclude that the y-coordinate will be constant ⇒ y2 = y1

Thus, by using distance formula:

PQ =

### Question 8. Find a point on the x-axis, which is equidistant from the point (7,6) and (3,4).

**Solution:**

As given in the question, let the arbitrary point C that lies on the x-axis has coordinate (x,0). Now, this point is equidistant from both the coordinates (7,6) and (3,4), therefore by using distance formula, we get

Therefore, coordinate of the point on the x-axis is

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