4.5
(100k+ )
1

### AIEEE 2005

If in a $$\Delta ABC$$, the altitudes from the vertices $$A, B, C$$ on opposite sides are in H.P, then $$\sin A,\sin B,\sin C$$ are in
A
G. P.
B
A. P.
C
A.P-G.P.
D
H. P

## Explanation

$$\Delta = {1 \over 2}{p_1}a = {1 \over 2}{p_2}b = {1 \over 2}{p_3}b$$

$${p_1},{p_2},{p_3},$$ are in $$H.P.$$

$$\Rightarrow {{2\Delta } \over a},{{2\Delta } \over b},{{2\Delta } \over c}$$ are in $$H.P.$$

$$\Rightarrow {1 \over a},{1 \over b},{1 \over c},$$ are in $$H.P.$$

$$\Rightarrow a,b,c$$ are in $$A.P.$$

$$\Rightarrow$$ $$K\sin A,K\sin B,K\sin C$$ are in $$A.P.$$

$$\Rightarrow$$ $$\sin A,\sin B,\sin C$$ are in $$A.P.$$
2

### AIEEE 2004

The sides of a triangle are $$\sin \alpha ,\,\cos \alpha$$ and $$\sqrt {1 + \sin \alpha \cos \alpha }$$ for some $$0 < \alpha < {\pi \over 2}$$. Then the greatest angle of the triangle is
A
$${150^ \circ }$$
B
$${90^ \circ }$$
C
$${120^ \circ }$$
D
$${60^ \circ }$$

## Explanation

Let $$a = \sin \alpha ,b = \cos \alpha$$

and $$c = \sqrt {1 + \sin \alpha \cos \alpha }$$

Clearly $$a$$ and $$b < 1$$ but $$c > 1$$

as $$\,\,\,\sin \alpha > 0$$ and $$\cos \alpha > 0$$

$$\therefore$$ $$c$$ is the greatest side and greatest angle is $$C$$

$$\therefore$$ $$\cos C = {{{a^2} + {b^2} - {c^2}} \over {2ab}}$$

$$= {{{{\sin }^2}\alpha + {{\cos }^2}\alpha - 1 - \sin \alpha \cos \alpha } \over {2\,\sin \alpha \cos \alpha }}$$

$$= - {1 \over 2}$$

$$\therefore$$ $$C = {120^ \circ }$$
3

### AIEEE 2004

A person standing on the bank of a river observes that the angle of elevation of the top of a tree on the opposite bank of the river is $${60^ \circ }$$ and when he retires $$40$$ meters away from the tree the angle of elevation becomes $${30^ \circ }$$. The breadth of the river is
A
$$60\,\,m$$
B
$$30\,\,m$$
C
$$40\,\,m$$
D
$$20\,\,m$$

## Explanation

From the figure

$$\tan {60^ \circ } = {y \over x}$$

$$\Rightarrow y = \sqrt {3x} .......\left( 1 \right)$$

$$\tan {30^ \circ } = {y \over {x + 40}}$$

$$\Rightarrow y = {{x + 40} \over {\sqrt 3 }}........\left( 2 \right)$$

From $$(1)$$ and $$(2),$$

$$\sqrt 3 x = {{x + 40} \over {\sqrt 3 }} \Rightarrow x = 20m$$
4

### AIEEE 2003

If in a $$\Delta ABC$$ $$a{\cos ^2}\left( {{C \over 2}} \right) + {\cos ^2}\left( {{A \over 2}} \right) = {{3b} \over 2},$$ then the sides $$a, b$$ and $$c$$
A
satisfy $$a+b=c$$
B
are in A.P
C
are in G.P
D
are in H.P

## Explanation

If $$a\,{\cos ^2}\left( {{C \over 2}} \right) + c\,{\cos ^2}\left( {{A \over 2}} \right) = {{3b} \over 2}$$

$$a\left[ {\cos C + 1} \right] + c\left[ {\cos A + 1} \right] = 3b$$

$$\left( {a + c} \right) + \left( {a\cos C + c\cos \,B} \right) = 3b$$

$$a + c + b = 3b$$ or $$a + c = 2b$$

or $$a,b,c$$ are in $$A.P.$$

### Joint Entrance Examination

JEE Main JEE Advanced WB JEE

### Graduate Aptitude Test in Engineering

GATE CSE GATE ECE GATE EE GATE ME GATE CE GATE PI GATE IN

NEET

Class 12