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1

WB JEE 2009

MCQ (Single Correct Answer)

The point ($$-$$4, 5) is the vertex of a square and one of its diagonals is 7x $$-$$ y + 8 = 0. The equation of the other diagonal is

A
7x $$-$$ y + 23 = 0
B
7y + x = 30
C
7y + x = 31
D
x $$-$$ 7y = 30

Explanation

Point ($$-$$4, 5) does not lie on the diagonal 7x $$-$$ y + 8 = 0, so point will lie on the other diagonal, also diagonals are perpendicular

$$\therefore$$ Slope of other diagonal = $$-$$1/7

$$\therefore$$ Equation of other diagonal is

$$y - 5 = - {1 \over 7}(x + 4) \Rightarrow 7y + x = 31$$

2

WB JEE 2009

MCQ (Single Correct Answer)

The number of points on the line x + y = 4 which are unit distance apart from the line 2x + 2y = 5 is

A
0
B
1
C
2
D
infinity

Explanation

Since x + y = 4 and 2x + 2y = 5 are parallel.

Take (4, 0) on the line x + y = 4

Distance of (4, 0) from the line 2x + 2y $$-$$ 5 = 0 is

$${{|2\,.\,4 + 2\,.\,0 - 5|} \over {\sqrt {{2^2} + {2^2}} }} = {3 \over {2\sqrt 2 }} = {{3\sqrt 2 } \over 4} > 1$$ (unit distance)

$$\because$$ Both lines are parallel and at a distance greater than unity

$$\therefore$$ There is no point on the line x + y = 4.

3

WB JEE 2009

MCQ (Single Correct Answer)

If C is the reflection of A(2, 4) in x-axis and B is the reflection of C in y-axis, then $$|AB|$$ is

A
20
B
2$$\sqrt5$$
C
4$$\sqrt5$$
D
4

Explanation

Reflection of point (h, k) about x-axis is (h, $$-$$k)

Reflection of point (h, k) about y-axis is ($$-$$h, k)

$$\therefore$$ Reflection point C of A(2, 4) in x-axis is (2, $$-$$4) and reflection point B of C(2, $$-$$4) in y-axis is ($$-$$2, $$-$$4)

$$\therefore$$ $$|AB| = \sqrt {{{(2 + 2)}^2} + {{(4 + 4)}^2}} = 4\sqrt 5 $$

4

WB JEE 2009

MCQ (Single Correct Answer)

A line through the point A(2, 0) which makes an angle of 30$$^\circ$$ with the positive direction of x-axis is rotated about A in clockwise direction through an angle 15$$^\circ$$. Then the equation of the straight line in the new position is

A
$$(2 - \sqrt 3 )x + y - 4 + 2\sqrt 3 = 0$$
B
$$(2 - \sqrt 3 )x - y - 4 + 2\sqrt 3 = 0$$
C
$$(2 - \sqrt 3 )x - y + 4 + 2\sqrt 3 = 0$$
D
$$(2 - \sqrt 3 )x + y + 4 + 2\sqrt 3 = 0$$

Explanation

Equation of line which is passing through (x1, y1) and angle of inclination $$\theta$$ with +ve x axis in anti-clockwise is

$$y - {y_1} = \tan \theta (x - {x_1})$$

$$\therefore$$ $$(y - 0) = \tan 15^\circ (x - 2)$$

$$[\tan 15^\circ = \tan (45^\circ - 30^\circ ) = {{\tan 45^\circ - \tan 30^\circ } \over {1 + \tan 45^\circ \tan 30^\circ }} = {{\sqrt 3 - 1} \over {\sqrt 3 + 1}} = 2 - \sqrt 3 ]$$

$$ \Rightarrow (2 - \sqrt 3 )x - y - 4 + 2\sqrt 3 = 0$$

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