Solutions to this question by accurate drawing will not be accepted. The diagram shows an isosceles triangle PQR in which PQ=QRPQ = QR. The vertices of the triangle are at the points P(k, 4), Q(5, 3) and R(9, 10).

  1. (i)

    Find the value of k.

  2. (ii)

    A line is drawn from Q to cut the y-axis at S such that PS=SRPS = SR.

    Find the equation of QS and the coordinates of S.

  3. (iii)

    Find the ratio of the area of triangle PQR to the area of quadrilateral PQRS.

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Worked solution

10(i)

1. Calculate the square of the length of QR

QR2=(9−5)2+(10−3)2QR^2 = (9 - 5)^2 + (10 - 3)^2
QR2=42+72=16+49=65QR^2 = 4^2 + 7^2 = 16 + 49 = 65

2. Express the square of the length of PQ in terms of k and equate to $QR^2$

Since PQ=QRPQ = QR, PQ2=QR2PQ^2 = QR^2.
PQ2=(k−5)2+(4−3)2PQ^2 = (k - 5)^2 + (4 - 3)^2
(k−5)2+12=65(k - 5)^2 + 1^2 = 65

3. Solve the quadratic equation for k

(k−5)2=64(k - 5)^2 = 64
k−5=±8k - 5 = \pm 8
k=5+8=13 or k=5−8=−3k = 5 + 8 = 13 \text{ or } k = 5 - 8 = -3

4. Determine the correct value of k based on the diagram

From the diagram, point P is to the left of the y-axis, so its x-coordinate must be negative (k<0k < 0).
Therefore, k=−3k = -3.

Answer

k=−3k = -3

10(ii)

1. Find the coordinates of P from part (i) using the given $PQ = QR$.

Using the distance formula for PQ2=QR2PQ^2 = QR^2:
(5−k)2+(3−4)2=(9−5)2+(10−3)2(5 - k)^2 + (3 - 4)^2 = (9 - 5)^2 + (10 - 3)^2
(5−k)2+1=16+49(5 - k)^2 + 1 = 16 + 49
(5−k)2=64(5 - k)^2 = 64
5−k=±85 - k = \pm 8
k=−3 or k=13k = -3 \text{ or } k = 13
From the diagram, P is to the left of the y-axis, so k<0k < 0. Thus, k=−3k = -3 and PP is (−3,4)(-3, 4).

2. Set up an equation for the coordinates of S using $PS = SR$.

Since S lies on the y-axis, let its coordinates be (0,y)(0, y).
Given PS=SRPS = SR, we have PS2=SR2PS^2 = SR^2:
(0−(−3))2+(y−4)2=(0−9)2+(y−10)2(0 - (-3))^2 + (y - 4)^2 = (0 - 9)^2 + (y - 10)^2
9+y2−8y+16=81+y2−20y+1009 + y^2 - 8y + 16 = 81 + y^2 - 20y + 100

3. Solve for y to find the coordinates of S.

25−8y=181−20y25 - 8y = 181 - 20y
12y=15612y = 156
y=13y = 13
The coordinates of S are (0,13)(0, 13).

4. Find the equation of the line QS.

Using Q(5,3)Q(5, 3) and S(0,13)S(0, 13):
Gradient of QS, m=13−30−5=10−5=−2m = \frac{13 - 3}{0 - 5} = \frac{10}{-5} = -2
Since S is the y-intercept, the equation of QS is:
y=−2x+13y = -2x + 13

Answer

Coordinates of S: (0,13)(0, 13)
Equation of QS: y=−2x+13y = -2x + 13

10(iii)

1. Find the coordinates of P and S from earlier parts

From part (i), PP is (−3,4)(-3, 4).
From part (ii), SS is (0,13)(0, 13).
The coordinates of the vertices are P(−3,4)P(-3, 4), Q(5,3)Q(5, 3), R(9,10)R(9, 10) and S(0,13)S(0, 13).

2. Calculate the area of triangle PQR

Using the shoelace formula for △PQR\triangle PQR with vertices P(−3,4)P(-3, 4), Q(5,3)Q(5, 3), R(9,10)R(9, 10):
Area=12∣(−3)(3)+(5)(10)+(9)(4)−(4)(5)−(3)(9)−(10)(−3)∣\text{Area} = \frac{1}{2} |(-3)(3) + (5)(10) + (9)(4) - (4)(5) - (3)(9) - (10)(-3)|
Area=12∣−9+50+36−20−27+30∣\text{Area} = \frac{1}{2} |-9 + 50 + 36 - 20 - 27 + 30|
Area=12∣60∣=30 units2\text{Area} = \frac{1}{2} |60| = 30 \text{ units}^2

3. Calculate the area of quadrilateral PQRS

Using the shoelace formula for quadrilateral PQRSPQRS with vertices P(−3,4)P(-3, 4), Q(5,3)Q(5, 3), R(9,10)R(9, 10), S(0,13)S(0, 13):
Area=12∣(−3)(3)+(5)(10)+(9)(13)+(0)(4)−(4)(5)−(3)(9)−(10)(0)−(13)(−3)∣\text{Area} = \frac{1}{2} |(-3)(3) + (5)(10) + (9)(13) + (0)(4) - (4)(5) - (3)(9) - (10)(0) - (13)(-3)|
Area=12∣−9+50+117+0−20−27−0+39∣\text{Area} = \frac{1}{2} |-9 + 50 + 117 + 0 - 20 - 27 - 0 + 39|
Area=12∣150∣=75 units2\text{Area} = \frac{1}{2} |150| = 75 \text{ units}^2

4. Find the ratio of the areas

Ratio=Area of △PQR:Area of PQRS\text{Ratio} = \text{Area of } \triangle PQR : \text{Area of } PQRS
Ratio=30:75\text{Ratio} = 30 : 75
Dividing by 15:
Ratio=2:5\text{Ratio} = 2 : 5

Answer

2:52 : 5

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