Question 37 of 480
From a point P, the bearings of two points Q and R are N670W and N230E respectively. If the bearing of R from Q is N680E and PQ = 150m, calculate PR
- A. 120m
- B. 140m
- C. 150m
- D. 160m
Correct Answer:
C
Explanation
To solve the problem, we need to visualize the situation and apply some trigonometry. Let's break it down step-by-step.
### Step 1: Understanding Bearings
Bearings are measured clockwise from the north direction.
- The bearing of point Q from point P is **N67°W**. This means if you start facing north and turn 67° towards the west, you will be facing point Q.
- The bearing of point R from point P is **N23°E**. This means if you start facing north and turn 23° towards the east, you will be facing point R.
- The bearing of point R from point Q is **N68°E**. This means if you are at point Q, you would turn 68° towards the east to face point R.
### Step 2: Drawing the Diagram
1. **Draw a vertical line** to represent the north direction.
2. **From point P**, draw a line towards Q at an angle of 67° to the west of north. This line represents PQ.
3. **From point P**, draw another line towards R at an angle of 23° to the east of north. This line represents PR.
4. **From point Q**, draw a line towards R at an angle of 68° to the east of north.
### Step 3: Labeling the Diagram
- Let’s denote the angle at point P between PQ and PR as angle \( \theta \).
- The angle between the north line and line PQ is 67°, and the angle between the north line and line PR is 23°.
- Therefore, the angle \( \theta \) at point P can be calculated as:
\[
\theta = 67° + 23° = 90°
\]
This means that the lines PQ and PR are perpendicular to each other.
### Step 4: Using the Right Triangle
Since we have established that PQ and PR are perpendicular, we can use the Pythagorean theorem to find the length of PR.
1. We know:
- \( PQ = 150 \, m \)
- \( QR \) can be found using the bearing from Q to R, which is N68°E.
2. To find \( QR \), we can use the sine rule or cosine rule, but since we have a right triangle, we can directly find the lengths using trigonometric ratios.
### Step 5: Finding PR
Using the right triangle formed by points P, Q, and R:
- We can use the sine of the angle at Q (which is 68°) to find QR:
\[
QR = PQ \cdot \tan(68°)
\]
However, we need to find PR directly. Since we have a right triangle, we can use the cosine rule:
\[
PR^2 = PQ^2 + QR^2
\]
But we need to find QR first.
### Step 6: Calculate QR
Using the sine rule:
\[
QR = PQ \cdot \tan(68°) = 150 \cdot \tan(68°)
\]
Calculating \( \tan(68°) \):
\[
\tan(68°) \approx 2.475
\]
Thus,
\[
QR \approx 150 \cdot 2.475 \approx 371.25 \, m
\]
### Step 7: Calculate PR
Now, we can find PR using the Pythagorean theorem:
\[
PR^2 = PQ^2 + QR^2
\]
Substituting the values:
\[
PR^2 = 150^2 + 371.25^2
\]
Calculating:
\[
PR^2 = 22500 + 137134.0625 \approx 159634.0625
\]
Taking the square root:
\[
PR \approx \sqrt{159634.0625} \approx 399.54 \, m
\]
### Conclusion
After calculating, we find that the length of PR is approximately 399.54 m, which does not match any of the options provided.
### Revision Summary
- Bearings are measured clockwise from north.
- Use trigonometric ratios to find lengths in right triangles.
- The Pythagorean theorem is useful for calculating distances in right triangles.
- Always double-check calculations for accuracy.
### Final Answer
The correct option is not listed among A, B, C, or D based on the calculations provided. Please verify the problem statement or the options given.