本题中通过精确作图得出的解答将不予采纳。 图(未按比例绘制)显示了一个梯形 OPQR,其中 PQPQ 平行于 OROR 且 igangleORQ=90∘igangle ORQ = 90^\circ。点 PP 和 RR 的坐标分别为 (−4, 3)(-4,\,3) 和 (4, 2)(4,\,2),且 OO 是原点。

  1. ((i))

    求 QQ 的坐标。

  2. ((ii))

    PQ 与 y 轴交于点 T。证明三角形 ORT 是等腰三角形。

  3. ((iii))

    点 S 满足 ORPS 构成一个平行四边形。

    求 S 的坐标。

  4. ((iv))

    求梯形 OPQR 的面积。

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完整解法

12((i))

1. Find the gradient of the line OR

Gradient of OR=2−04−0=12OR = \frac{2 - 0}{4 - 0} = \frac{1}{2}

2. Find the equation of the line PQ

Since PQ∥ORPQ \parallel OR, gradient of PQ=12PQ = \frac{1}{2}.
Using point P(−4,3)P(-4, 3):
y−3=12(x−(−4))y - 3 = \frac{1}{2}(x - (-4))
y=12x+2+3y = \frac{1}{2}x + 2 + 3
y=12x+5y = \frac{1}{2}x + 5

3. Find the gradient of the line RQ

Since ∠ORQ=90∘\angle ORQ = 90^\circ, RQ⊥ORRQ \perp OR.
Gradient of RQ=−112=−2RQ = -\frac{1}{\frac{1}{2}} = -2

4. Find the equation of the line RQ

Using point R(4,2)R(4, 2):
y−2=−2(x−4)y - 2 = -2(x - 4)
y=−2x+8+2y = -2x + 8 + 2
y=−2x+10y = -2x + 10

5. Solve the equations simultaneously to find the coordinates of Q

Equating the two expressions for yy:
12x+5=−2x+10\frac{1}{2}x + 5 = -2x + 10
52x=5\frac{5}{2}x = 5
x=2x = 2

Substitute x=2x = 2 into y=−2x+10y = -2x + 10:
y=−2(2)+10=6y = -2(2) + 10 = 6

The coordinates of QQ are (2,6)(2, 6).

答案

(2,6)(2, 6)

12((ii))

1. Find the gradient of OR and the equation of line PQ.

Gradient of OR=2−04−0=12OR = \frac{2 - 0}{4 - 0} = \frac{1}{2}.
Since PQ∥ORPQ \parallel OR, gradient of PQ=12PQ = \frac{1}{2}.
Equation of PQPQ passing through P(−4,3)P(-4, 3):
y−3=12(x−(−4))y - 3 = \frac{1}{2}(x - (-4))
y=12x+2+3y = \frac{1}{2}x + 2 + 3
y=12x+5y = \frac{1}{2}x + 5

2. Find the coordinates of T.

Since TT is the y-intercept of PQPQ, let x=0x = 0.
y=12(0)+5=5y = \frac{1}{2}(0) + 5 = 5.
Coordinates of TT are (0,5)(0, 5).

3. Calculate the lengths of OT, OR, and RT to show two sides are equal.

Length of OT=5OT = 5 units.
Length of OR=(4−0)2+(2−0)2=16+4=20OR = \sqrt{(4-0)^2 + (2-0)^2} = \sqrt{16 + 4} = \sqrt{20}.
Length of RT=(4−0)2+(2−5)2=16+(−3)2=16+9=25=5RT = \sqrt{(4-0)^2 + (2-5)^2} = \sqrt{16 + (-3)^2} = \sqrt{16 + 9} = \sqrt{25} = 5.
Since OT=RT=5OT = RT = 5, triangle ORTORT is isosceles.

答案

Triangle ORT is isosceles since OT=RT=5OT = RT = 5.

12((iii))

1. Equate the midpoints of the diagonals of the parallelogram

In parallelogram ORPSORPS, the diagonals OPOP and RSRS bisect each other. Therefore, their midpoints are the same.
Let the coordinates of SS be (x,y)(x, y).
Midpoint of OPOP = Midpoint of RSRS
(0+(−4)2,0+32)=(4+x2,2+y2)\left( \frac{0 + (-4)}{2}, \frac{0 + 3}{2} \right) = \left( \frac{4 + x}{2}, \frac{2 + y}{2} \right)

2. Solve for the coordinates of S

Equating the xx-coordinates:
−42=4+x2\frac{-4}{2} = \frac{4 + x}{2}
−4=4+x  ⟹  x=−8-4 = 4 + x \implies x = -8

Equating the yy-coordinates:
32=2+y2\frac{3}{2} = \frac{2 + y}{2}
3=2+y  ⟹  y=13 = 2 + y \implies y = 1

The coordinates of SS are (−8,1)(-8, 1).

答案

(−8,1)(-8, 1)

12((iv))

1. Apply the shoelace formula using the coordinates of the vertices in order

Using the coordinates O(0,0)O(0,0), R(4,2)R(4,2), Q(2,6)Q(2,6) (from part (i)), and P(−4,3)P(-4,3) in counter-clockwise order:
Area=12∣042−4002630∣\text{Area} = \frac{1}{2} \left| \begin{matrix} 0 & 4 & 2 & -4 & 0 \\ 0 & 2 & 6 & 3 & 0 \end{matrix} \right|
Area=12∣(0×2+4×6+2×3+(−4)×0)−(0×4+2×2+6×(−4)+3×0)∣\text{Area} = \frac{1}{2} |(0 \times 2 + 4 \times 6 + 2 \times 3 + (-4) \times 0) - (0 \times 4 + 2 \times 2 + 6 \times (-4) + 3 \times 0)|

2. Evaluate the expression to find the area

Area=12∣(0+24+6+0)−(0+4−24+0)∣\text{Area} = \frac{1}{2} |(0 + 24 + 6 + 0) - (0 + 4 - 24 + 0)|
Area=12∣30−(−20)∣\text{Area} = \frac{1}{2} |30 - (-20)|
Area=12(50)=25\text{Area} = \frac{1}{2} (50) = 25

答案

2525 units2^2

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