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Trigonometry

Important Instructions
71.

If the length of the shadow of a tower is increasing, then the angle of elevation of the sun is:

Answer: B

Let h be the height of the tower and s be the length of the shadow. The angle of elevation θ is given by \(\tan \theta = h/s\).

Since the height h is constant, if the shadow length 's' increases, the value of the fraction h/s decreases.

As \(\tan \theta\) decreases, the angle θ also decreases (for acute angles). Therefore, the angle of elevation is decreasing.

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72.

If \(\sin \theta = \frac{3}{5}\), what is the value of \(\cos \theta\)?

Answer: B

We use the fundamental trigonometric identity \(\sin^2 \theta + \cos^2 \theta = 1\).

\((\frac{3}{5})^2 + \cos^2 \theta = 1\)

\(\frac{9}{25} + \cos^2 \theta = 1\)

\(\cos^2 \theta = 1 - \frac{9}{25} = \frac{16}{25}\)

\(\cos \theta = \sqrt{\frac{16}{25}} = \frac{4}{5}\) (Assuming θ is in the first quadrant).

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73.

A person standing on the bank of a river observes that the angle subtended by a tree on the opposite bank is 60°. When he retreats 40 m from the bank, he finds the angle to be 30°. The height of the tree is:

Answer: B

Let h be the height of the tree and x be the initial distance from the tree.

From the first observation: \(\tan 60° = \frac{h}{x} \Rightarrow \sqrt{3} = \frac{h}{x} \Rightarrow h = x\sqrt{3}\).

From the second observation: \(\tan 30° = \frac{h}{x+40} \Rightarrow \frac{1}{\sqrt{3}} = \frac{h}{x+40} \Rightarrow x+40 = h\sqrt{3}\).

Substitute h from the first equation into the second: \(x+40 = (x\sqrt{3})\sqrt{3} = 3x\).

\(40 = 2x \Rightarrow x = 20\) m.

Now find the height: \(h = x\sqrt{3} = 20\sqrt{3}\) m.

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74.

The value of \(\sin(2A)\) is given by:

Answer: C

This is the standard double-angle identity for sine.

\(\sin(2A) = 2\sin A \cos A\).

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75.

If a triangle ABC is right angled at C, then the value of \(\cos(A+B)\) is:

Answer: A

In any triangle, A + B + C = 180°.

Since the triangle is right-angled at C, we have C = 90°.

A + B + 90° = 180°

A + B = 90°.

Therefore, \(\cos(A+B) = \cos(90°) = 0\).

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76.

What is the minimum value of \(\sin^2\theta + \cos^2\theta\)?

Answer: B

This is a trick question based on the fundamental Pythagorean identity.

For any angle \(\theta\), the expression \(\sin^2\theta + \cos^2\theta\) is always equal to 1.

Therefore, its minimum value, maximum value, and average value are all 1.

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77.

The top of two poles of height 20 m and 14 m are connected by a wire. If the wire makes an angle of 30° with the horizontal, then the length of the wire is:

Answer: D

Imagine a right-angled triangle formed by the wire (hypotenuse), the horizontal distance, and the difference in the heights of the poles (opposite side).

The difference in height = 20 m - 14 m = 6 m.

The angle of the wire with the horizontal is 30°.

\(\sin 30° = \frac{\text{Opposite}}{\text{Hypotenuse}} = \frac{\text{Height Difference}}{\text{Wire Length}}\)

\(\frac{1}{2} = \frac{6}{\text{Wire Length}}\)

Wire Length = 6 × 2 = 12 meters.

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78.

The value of \(\frac{1-\tan^2 45°}{1+\tan^2 45°}\) is:

Answer: D

We know that \(\tan 45° = 1\).

Substitute this value into the expression:

\(\frac{1 - (1)^2}{1 + (1)^2} = \frac{1 - 1}{1 + 1} = \frac{0}{2} = 0\).

Alternatively, this is the formula for \(\cos(2\theta)\), so it equals \(\cos(2 \times 45°) = \cos 90° = 0\).

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79.

If \(\sin(A - B) = 1/2\) and \(\cos(A + B) = 1/2\), the value of B is:

Answer: A

From the given information:

1) \(\sin(A - B) = 1/2 \Rightarrow A - B = 30°\)

2) \(\cos(A + B) = 1/2 \Rightarrow A + B = 60°\)

We have two equations. To find B, subtract the first equation from the second:

(A + B) - (A - B) = 60° - 30°

2B = 30°

B = 15°.

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80.

A ladder 15 m long just reaches the top of a vertical wall. If the ladder makes an angle of 60° with the wall, then the height of the wall is:

Answer: C

Be careful with the angle. The angle is with the *wall*, not the ground.

Let h be the height of the wall (adjacent side to the 60° angle) and L be the length of the ladder (hypotenuse).

\(\cos(\text{angle}) = \frac{\text{Adjacent}}{\text{Hypotenuse}}\)

\(\cos 60° = \frac{h}{15}\)

\(\frac{1}{2} = \frac{h}{15}\)

h = 15 / 2 = 7.5 meters.

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Tags: Trigonometry Questions and Answers || Trigonometry MCQ Questions || Trigonometric Functions Quantitative Aptitude || Trigonometry GK Questions || Quantitative Aptitude Trigonometry