PP、QQ、RR 和 SS 是位于一个岛上的四个村庄,其中 QQ 位于 RR 的正北方。
PQ=924 mPQ = 924\text{ m},QR=640 mQR = 640\text{ m},PR=1124 mPR = 1124\text{ m},RS=583 mRS = 583\text{ m}。
角 RPS=26∘RPS = 26^\circ 且角 PSRPSR 为钝角。

  1. (a)

    证明 PP 位于 QQ 的正东方。

    答

  2. (b)

    求 SS 相对于 RR 的方位角。

    答 ... ∘^\circ

  3. (c)

    一位村民以 9.5 km/h 的恒定速度从村庄 QQ 慢跑至村庄 SS,他走的是最短路线。求该村民从村庄 QQ 慢跑至村庄 SS 所用的时间。用分钟和秒表示你的答案,精确到最接近的十秒。

    答 ... 分 ... 秒

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完整解法

4(a)

1. Calculate the sum of the squares of the two shorter sides of triangle PQR.

PQ2+QR2=9242+6402PQ^2 + QR^2 = 924^2 + 640^2
=853776+409600= 853776 + 409600
=1263376= 1263376

2. Calculate the square of the longest side and compare.

PR2=11242=1263376PR^2 = 1124^2 = 1263376
Since PQ2+QR2=PR2PQ^2 + QR^2 = PR^2, by the converse of Pythagoras' theorem, △PQR\triangle PQR is a right-angled triangle with ∠PQR=90∘\angle PQR = 90^\circ.

3. Use the bearing information to conclude the relative position of P.

We are given that QQ is due north of RR, which means the line segment QRQR lies on the North-South axis.
Since ∠PQR=90∘\angle PQR = 90^\circ and from the given layout PP is to the right of the North-South line QRQR, PP must be due east of QQ.

答案

Since PQ2+QR2=PR2PQ^2 + QR^2 = PR^2, ∠PQR=90∘\angle PQR = 90^\circ. As QQ is due north of RR, PP is due east of QQ.

4(b)

1. Find angle PRQ using right-angled trigonometry

In △PQR\triangle PQR, since PP is due east of QQ and QQ is due north of RR, ∠PQR=90∘\angle PQR = 90^\circ.
tan⁡(∠PRQ)=PQQR=924640\tan(\angle PRQ) = \frac{PQ}{QR} = \frac{924}{640}
∠PRQ=tan⁡−1(924640)≈55.291∘\angle PRQ = \tan^{-1}\left(\frac{924}{640}\right) \approx 55.291^\circ

2. Find the obtuse angle PSR using the Sine Rule

In △PRS\triangle PRS, using the Sine Rule:
sin⁡(∠PSR)1124=sin⁡26∘583\frac{\sin(\angle PSR)}{1124} = \frac{\sin 26^\circ}{583}
sin⁡(∠PSR)=1124sin⁡26∘583≈0.84516\sin(\angle PSR) = \frac{1124 \sin 26^\circ}{583} \approx 0.84516
Since ∠PSR\angle PSR is obtuse:
∠PSR=180∘−sin⁡−1(0.84516)≈180∘−57.689∘=122.311∘\angle PSR = 180^\circ - \sin^{-1}(0.84516) \approx 180^\circ - 57.689^\circ = 122.311^\circ

3. Find angle PRS

Using the sum of angles in △PRS\triangle PRS:
∠PRS=180∘−26∘−122.311∘=31.689∘\angle PRS = 180^\circ - 26^\circ - 122.311^\circ = 31.689^\circ

4. Calculate the bearing of S from R

Since QQ is due north of RR, the bearing of SS from RR is the angle QRSQRS.
∠QRS=∠PRQ+∠PRS=55.291∘+31.689∘=86.980∘\angle QRS = \angle PRQ + \angle PRS = 55.291^\circ + 31.689^\circ = 86.980^\circ
Bearing =087.0∘= 087.0^\circ (to 1 d.p.)

答案

087.0∘087.0^\circ

4(c)

1. Find angle $PRS$ using the sine rule in triangle $PRS$.

Using the sine rule in △PRS\triangle PRS:
sin⁡∠PSR1124=sin⁡26∘583\frac{\sin \angle PSR}{1124} = \frac{\sin 26^\circ}{583}
sin⁡∠PSR=1124sin⁡26∘583≈0.8451\sin \angle PSR = \frac{1124 \sin 26^\circ}{583} \approx 0.8451
Since ∠PSR\angle PSR is obtuse:
∠PSR=180∘−sin⁡−1(0.8451)≈122.317∘\angle PSR = 180^\circ - \sin^{-1}(0.8451) \approx 122.317^\circ
∠PRS=180∘−26∘−122.317∘=31.683∘\angle PRS = 180^\circ - 26^\circ - 122.317^\circ = 31.683^\circ

2. Find angle $QRS$ by adding angle $PRQ$ and angle $PRS$.

In right-angled △PQR\triangle PQR (since PP is due east of QQ and QQ is due north of RR):
tan⁡∠PRQ=924640\tan \angle PRQ = \frac{924}{640}
∠PRQ=tan⁡−1(924640)≈55.292∘\angle PRQ = \tan^{-1}\left(\frac{924}{640}\right) \approx 55.292^\circ
∠QRS=∠PRQ+∠PRS=55.292∘+31.683∘=86.975∘\angle QRS = \angle PRQ + \angle PRS = 55.292^\circ + 31.683^\circ = 86.975^\circ

3. Calculate the distance $QS$ using the cosine rule in triangle $QRS$.

Using the cosine rule in △QRS\triangle QRS:
QS2=QR2+RS2−2(QR)(RS)cos⁡∠QRSQS^2 = QR^2 + RS^2 - 2(QR)(RS)\cos \angle QRS
QS2=6402+5832−2(640)(583)cos⁡86.975∘QS^2 = 640^2 + 583^2 - 2(640)(583)\cos 86.975^\circ
QS2=409600+339889−746240(0.05276)≈710115QS^2 = 409600 + 339889 - 746240(0.05276) \approx 710115
QS=710115≈842.68 mQS = \sqrt{710115} \approx 842.68 \text{ m}

4. Calculate the time taken to jog from $Q$ to $S$ and round to the nearest ten seconds.

Speed =9.5 km/h=95003600 m/s≈2.6389 m/s= 9.5 \text{ km/h} = \frac{9500}{3600} \text{ m/s} \approx 2.6389 \text{ m/s}
Time taken =DistanceSpeed=842.682.6389≈319.33 s= \frac{\text{Distance}}{\text{Speed}} = \frac{842.68}{2.6389} \approx 319.33 \text{ s}
319.33 s=5 min 19.33 s319.33 \text{ s} = 5 \text{ min } 19.33 \text{ s}
Correct to the nearest ten seconds, the time is 5 min 20 s5 \text{ min } 20 \text{ s}.

答案

5 min 20 s

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