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Some Applications Of Trigonometry
EXAMPLES
CBSE_NCERT_TEXTBOOK
Grade 10
Question:
An electrician has to repair an electric fault on a pole of height 5 m. She needs to reach a point 1.3m below the top of the pole to undertake the repair work (see Fig. 9.5). What should be the length of the ladder that she should use which, when inclined at an angle of 60° to the horizontal, would enable her to reach the required position? Also, how far from the foot of the pole should she place the foot of the ladder? (You may take 3 = 1.73)
Step-by-Step Solution
Key Concept: Use the basic trigonometric ratios for a right‑angled triangle: \(\sin\theta = \frac{\text{opposite}}{\text{hypotenuse}}\), \(\cos\theta = \frac{\text{adjacent}}{\text{hypotenuse}}\) and \(\tan\theta = \frac{\text{opposite}}{\text{adjacent}}\). Here the ladder, the ground and the pole form a right‑angled triangle with the ladder as the hypotenuse, the vertical distance to be reached as the opposite side and the horizontal distance from the pole as the adjacent side.
1. Determine the vertical distance to be reached. The pole is 5 m high and the electrician must work 1.3 m below the top, so the required vertical height from the ground is $$h = 5 - 1.3 = 3.7\ \text{m}.$$ 2. Use the given angle (60°) with the horizontal. The ladder makes an angle \(\theta = 60^{\circ}\) with the ground. 3. Find the length of the ladder (hypotenuse). Using the sine ratio: $$\sin 60^{\circ} = \frac{\text{opposite}}{\text{hypotenuse}} = \frac{h}{L}.$$ Since \(\sin 60^{\circ} = \frac{\sqrt{3}}{2}\) and \(\sqrt{3} \approx 1.73\), $$\sin 60^{\circ} = \frac{1.73}{2} = 0.865.$$ Hence $$L = \frac{h}{\sin 60^{\circ}} = \frac{3.7}{0.865} \approx 4.28\ \text{m}.$$ (Exact form: \(L = \frac{3.7 \times 2}{\sqrt{3}} = \frac{7.4}{\sqrt{3}} = \frac{7.4\sqrt{3}}{3}\).) 4. Find the horizontal distance from the foot of the ladder to the pole. Using the cosine ratio or tangent ratio: - Cosine method: $$\cos 60^{\circ} = \frac{\text{adjacent}}{\text{hypotenuse}} = \frac{d}{L} \;\Rightarrow\; d = L\cos 60^{\circ} = L \times \frac{1}{2} = \frac{L}{2}.$$ Substituting \(L \approx 4.28\) gives $$d \approx \frac{4.28}{2} = 2.14\ \text{m}.$$ - Tangent method (check): $$\tan 60^{\circ} = \frac{\text{opposite}}{\text{adjacent}} = \frac{h}{d} \;\Rightarrow\; d = \frac{h}{\tan 60^{\circ}} = \frac{3.7}{1.73} \approx 2.14\ \text{m}.$$ 5. State the final answers. - Length of ladder required: approximately \(4.28\) m (or \(4.3\) m to one decimal place). - Distance of the foot of the ladder from the pole: approximately \(2.14\) m (or \(2.1\) m to one decimal place).
Correct Answer:Length of ladder ≈ 4.28 m (≈ 4.3 m). Distance of foot from pole ≈ 2.14 m (≈ 2.1 m).
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