一名土木工程师正在设计一座长 131 米、高 40 米的桥梁,桥身将包含四个完全相同的抛物线拱。每个拱高 34 米,相邻两拱的底端之间相距 1 米,如图所示。建立坐标系,以第一个拱底端的左端点为原点。

  1. (a)

    求拱 1 的 xx 轴截距。

  2. (b)

    表示拱 1 的方程可以写成 y=a(x−p)(x−q)y = a(x - p)(x - q) 的形式。证明 a=−17128a = -\frac{17}{128}。

  3. (c)

    通过计算并说明,点 (50,30)(50, 30) 是否在拱 2 上。

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完整解法

8(a)

1. Find the width of a single arch

Let ww be the width of one arch.
The total length of the bridge consists of 4 arches and 3 gaps of 1 m each.
4w+3(1)=1314w + 3(1) = 131
4w=1284w = 128
w=32w = 32

2. Determine the $x$-intercepts of Arch 1

Since Arch 1 starts at the origin (x=0x = 0) and has a width of 3232 m, its xx-intercepts are at x=0x = 0 and x=32x = 32.

答案

00 and 3232

8(b)

1. Substitute the $x$-intercepts of Arch 1 into the factored form of the quadratic equation.

From part (a), the xx-intercepts of Arch 1 are 00 and 3232.
Thus, p=0p = 0 and q=32q = 32.
The equation is y=a(x−0)(x−32)=ax(x−32)y = a(x - 0)(x - 32) = ax(x - 32).

2. Use the maximum height of the arch to find the value of $a$.

The maximum height occurs at the midpoint of the xx-intercepts, which is x=0+322=16x = \frac{0 + 32}{2} = 16.
The height of the arch is 3434 m, so the vertex is at (16,34)(16, 34).
Substitute x=16x = 16 and y=34y = 34 into the equation:
34=a(16)(16−32)34 = a(16)(16 - 32)
34=a(16)(−16)34 = a(16)(-16)
34=−256a34 = -256a
a=−34256=−17128a = -\frac{34}{256} = -\frac{17}{128} (shown)

答案

Shown that a=−17128a = -\frac{17}{128}.

8(c)

1. Determine the $x$-intercepts of Arch 1 from the context

Let the width of each arch be ww. There are 4 arches and 3 gaps of 1 m each.
4w+3(1)=1314w + 3(1) = 131
4w=128  ⟹  w=324w = 128 \implies w = 32
The xx-intercepts of Arch 1 are x=0x = 0 and x=32x = 32.

2. Determine the $x$-intercepts of Arch 2

Arch 2 starts after Arch 1 and a 1 m gap.
Start of Arch 2: x=32+1=33x = 32 + 1 = 33
End of Arch 2: x=33+32=65x = 33 + 32 = 65
The xx-intercepts of Arch 2 are x=33x = 33 and x=65x = 65.

3. Form the equation of Arch 2

Since the arches are identical, the value of aa is the same as Arch 1, a=−17128a = -\frac{17}{128}.
The equation of Arch 2 is y=−17128(x−33)(x−65)y = -\frac{17}{128}(x - 33)(x - 65).

4. Check if the point $(50, 30)$ satisfies the equation of Arch 2

Substitute x=50x = 50 into the equation of Arch 2:
y=−17128(50−33)(50−65)y = -\frac{17}{128}(50 - 33)(50 - 65)
y=−17128(17)(−15)y = -\frac{17}{128}(17)(-15)
y=4335128=33.867...y = \frac{4335}{128} = 33.867...
Since y≠30y \neq 30, the point (50,30)(50, 30) does not lie on Arch 2.

答案

No, the point (50,30)(50, 30) does not lie on Arch 2.

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