图中显示了 ΔABC。

  1. (i)

    May说直线 AC 与 y 轴相交于 3.2。请展示你会如何检验她的说法是否正确。

    答

  2. (ii)

    求 AB 的长度。

    答:___ 单位

  3. (iii)

    在不计算任何角的值的情况下,求 cos ÂBC 的值。

    答:cos ÂBC = ___

  4. (iv)

    点 P 在直线 y = 1 上。已知 ΔABC 的面积 : ΔAPB 的面积 = 4: 3,求 P 的两个可能的 x 坐标。

    答:x = ___ 或 ___

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完整解法

10 (a)(i)

1. Find the gradient of line AC from the two given coordinates.

For A(−2,4)A(-2,4) and C(6,1)C(6,1),
m=1−46−(−2)=−38.m=\frac{1-4}{6-(-2)}=\frac{-3}{8}.

2. Find the y-intercept of AC and check May's claim.

Using y=mx+cy=mx+c and point A(−2,4)A(-2,4),
4=−38(−2)+c4=-\frac{3}{8}(-2)+c
4=34+c4=\frac{3}{4}+c
c=4−34=134=3.25.c=4-\frac{3}{4}=\frac{13}{4}=3.25.
So the line intersects the yy-axis at 3.253.25.

答案

May is incorrect, since line ACAC intersects the yy-axis at 3.253.25, not 3.23.2.

10 (a)(ii)

1. Find the horizontal and vertical differences between A and B.

A(−2,4)A(-2,4) and B(2,1)B(2,1)

Horizontal difference: 2−(−2)=42-(-2)=4

Vertical difference: 1−4=−31-4=-3, so the vertical distance is 33.

2. Apply the distance formula to find AB.

AB=42+(−3)2AB=\sqrt{4^2+(-3)^2}
=16+9=\sqrt{16+9}
=25=5=\sqrt{25}=5

答案

AB=5AB = 5 units

10 (a)(iii)

1. Identify the adjacent side and hypotenuse for the angle at $B$, noting that the angle is obtuse.

From B(2,1)B(2,1) to A(−2,4)A(-2,4), the horizontal change is 44 units left and the vertical change is 33 units up, so AB=5AB=5 units.

Since BCBC is horizontal to the right, ∠ABC\angle ABC is obtuse. Hence
cos⁡∠ABC=−45.\cos \angle ABC = -\frac{4}{5}.

答案

cos⁡∠ABC=−45\cos \angle ABC = -\dfrac{4}{5}

10 (a)(iv)

1. Compare the areas using the bases on the same horizontal line.

Since BB, CC and PP lie on y=1y=1, triangles ABCABC and APBAPB have the same perpendicular height from AA.

Therefore,
area of △ABCarea of △APB=BCBP.\frac{\text{area of }\triangle ABC}{\text{area of }\triangle APB}=\frac{BC}{BP}.
Given
area of △ABC:area of △APB=4:3,\text{area of }\triangle ABC : \text{area of }\triangle APB = 4:3,
so
BC:BP=4:3.BC:BP=4:3.

2. Find the possible positions of $P$ from the required length $BP$.

From the coordinates, B=(2,1)B=(2,1) and C=(6,1)C=(6,1), so
BC=6−2=4.BC=6-2=4.
Since
BC:BP=4:3,BC:BP=4:3,
BP=3.BP=3.

As PP lies on y=1y=1, it is 3 units horizontally from BB, whose xx-coordinate is 2.

Hence
x=2−3=−1x=2-3=-1
or
x=2+3=5.x=2+3=5.

答案

x=−1x=-1 or x=5x=5

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