平面 p1p_1 和 p2p_2 的方程分别由下式给出 r=s(011)+t(3−12),s,t∈Randr⋅(α3β)=7,\mathbf{r} = s \begin{pmatrix} 0 \\ 1 \\ 1 \end{pmatrix} + t \begin{pmatrix} 3 \\ -1 \\ 2 \end{pmatrix}, \quad s, t \in \mathbb{R} \quad \text{and} \quad \mathbf{r} \cdot \begin{pmatrix} \alpha \\ 3 \\ \beta \end{pmatrix} = 7, 其中 α\alpha 和 β\beta 为实常数。

  1. (a)

    已知直线 l1:x+14=2−y3=zl_1: \frac{x+1}{4} = \frac{2-y}{3} = z 在 p2p_2 内,求 α\alpha 和 β\beta 的值。

  2. (b)

    现已知 α=−3\alpha = -3 且 β=2\beta = 2。

    求 p1p_1 和 p2p_2 的交线方程。

  3. (c)

    方程为 y+3z+4=0y + 3z + 4 = 0 的平面 p3p_3 平行于 l1l_1。求 l1l_1 和 p3p_3 之间的距离。

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完整解法

4(a)

1. Extract a point and the direction vector from the Cartesian equation of line $l_1$.

Rewrite the equation of l1l_1 into the standard form x−x0a=y−y0b=z−z0c\frac{x - x_0}{a} = \frac{y - y_0}{b} = \frac{z - z_0}{c}:

l1:x+14=y−2−3=z−01l_1: \frac{x+1}{4} = \frac{y-2}{-3} = \frac{z-0}{1}

From this, a point on l1l_1 has position vector a=(−120)\mathbf{a} = \begin{pmatrix} -1 \\ 2 \\ 0 \end{pmatrix} and the direction vector of l1l_1 is d=(4−31)\mathbf{d} = \begin{pmatrix} 4 \\ -3 \\ 1 \end{pmatrix}.

2. Substitute the point on $l_1$ into the equation of plane $p_2$ to find $\alpha$.

Since the line l1l_1 lies completely in plane p2p_2, any point on l1l_1 must also lie on p2p_2. Substituting the point (−1,2,0)(-1, 2, 0) into the equation of p2p_2:

(−120)⋅(α3β)=7\begin{pmatrix} -1 \\ 2 \\ 0 \end{pmatrix} \cdot \begin{pmatrix} \alpha \\ 3 \\ \beta \end{pmatrix} = 7

−α+6+0=7-\alpha + 6 + 0 = 7
−α=1-\alpha = 1
α=−1\alpha = -1

3. Use the perpendicularity between the line's direction vector and the plane's normal vector to find $\beta$.

Since l1l_1 lies in p2p_2, its direction vector d\mathbf{d} must be perpendicular to the normal vector n2\mathbf{n_2} of p2p_2. Hence, their dot product is zero:

d⋅n2=0\mathbf{d} \cdot \mathbf{n_2} = 0
(4−31)⋅(−13β)=0\begin{pmatrix} 4 \\ -3 \\ 1 \end{pmatrix} \cdot \begin{pmatrix} -1 \\ 3 \\ \beta \end{pmatrix} = 0

−4−9+β=0-4 - 9 + \beta = 0
−13+β=0-13 + \beta = 0
β=13\beta = 13

答案

α=−1\alpha = -1, β=13\beta = 13

4(b)

1. Substitute the parametric form of $p_1$ into the scalar product equation of $p_2$ to find the relationship between parameters $s$ and $t$.

From the equation of p1p_1, a generic point on the plane has position vector:
r=(3ts−ts+2t)\mathbf{r} = \begin{pmatrix} 3t \\ s - t \\ s + 2t \end{pmatrix}
Substituting this into the equation of p2p_2:
(3ts−ts+2t)⋅(−332)=7\begin{pmatrix} 3t \\ s - t \\ s + 2t \end{pmatrix} \cdot \begin{pmatrix} -3 \\ 3 \\ 2 \end{pmatrix} = 7
−3(3t)+3(s−t)+2(s+2t)=7-3(3t) + 3(s - t) + 2(s + 2t) = 7
−9t+3s−3t+2s+4t=7-9t + 3s - 3t + 2s + 4t = 7
5s−8t=75s - 8t = 7

2. Express $s$ in terms of $t$ and substitute it back into the equation of $p_1$.

s=8t+75s = \frac{8t + 7}{5}
Substituting ss back into the equation of p1p_1:
r=(8t+75)(011)+t(3−12)\mathbf{r} = \left( \frac{8t + 7}{5} \right) \begin{pmatrix} 0 \\ 1 \\ 1 \end{pmatrix} + t \begin{pmatrix} 3 \\ -1 \\ 2 \end{pmatrix}
r=(07575)+8t5(011)+t(3−12)\mathbf{r} = \begin{pmatrix} 0 \\ \frac{7}{5} \\ \frac{7}{5} \end{pmatrix} + \frac{8t}{5} \begin{pmatrix} 0 \\ 1 \\ 1 \end{pmatrix} + t \begin{pmatrix} 3 \\ -1 \\ 2 \end{pmatrix}
r=(07575)+t5(158−58+10)\mathbf{r} = \begin{pmatrix} 0 \\ \frac{7}{5} \\ \frac{7}{5} \end{pmatrix} + \frac{t}{5} \begin{pmatrix} 15 \\ 8 - 5 \\ 8 + 10 \end{pmatrix}
r=(07575)+t5(15318)\mathbf{r} = \begin{pmatrix} 0 \\ \frac{7}{5} \\ \frac{7}{5} \end{pmatrix} + \frac{t}{5} \begin{pmatrix} 15 \\ 3 \\ 18 \end{pmatrix}

3. Simplify the direction vector to obtain the standard vector equation of the line.

Factoring out 33 from the direction vector components:
r=(07575)+3t5(516)\mathbf{r} = \begin{pmatrix} 0 \\ \frac{7}{5} \\ \frac{7}{5} \end{pmatrix} + \frac{3t}{5} \begin{pmatrix} 5 \\ 1 \\ 6 \end{pmatrix}
Letting λ=3t5\lambda = \frac{3t}{5}, the line of intersection is:
r=(07575)+λ(516),λ∈R\mathbf{r} = \begin{pmatrix} 0 \\ \frac{7}{5} \\ \frac{7}{5} \end{pmatrix} + \lambda \begin{pmatrix} 5 \\ 1 \\ 6 \end{pmatrix}, \quad \lambda \in \mathbb{R}

答案

r=(07575)+λ(516),λ∈R\mathbf{r} = \begin{pmatrix} 0 \\ \frac{7}{5} \\ \frac{7}{5} \end{pmatrix} + \lambda \begin{pmatrix} 5 \\ 1 \\ 6 \end{pmatrix}, \quad \lambda \in \mathbb{R}

4(c)

1. Identify the coordinates of a known point on the line $l_1$.

From the equation of l1:x+14=y−2−3=z1l_1: \frac{x+1}{4} = \frac{y-2}{-3} = \frac{z}{1}, a point on the line is A(−1,2,0)A(-1, 2, 0).

2. Calculate the perpendicular distance from point $A$ to plane $p_3$.

Since l1l_1 is parallel to p3p_3, the distance between them is the perpendicular distance from any point on l1l_1 to p3p_3.

The equation of p3p_3 can be expressed as r⋅(013)=−4\mathbf{r} \cdot \begin{pmatrix} 0 \\ 1 \\ 3 \end{pmatrix} = -4.
Using the distance formula ∣a⋅n−d∣∣n∣\frac{|\mathbf{a} \cdot \mathbf{n} - d|}{|\mathbf{n}|}:

Distance=∣(−120)⋅(013)−(−4)∣02+12+32\text{Distance} = \frac{\left| \begin{pmatrix} -1 \\ 2 \\ 0 \end{pmatrix} \cdot \begin{pmatrix} 0 \\ 1 \\ 3 \end{pmatrix} - (-4) \right|}{\sqrt{0^2 + 1^2 + 3^2}}
Distance=∣(0+2+0)+4∣10\text{Distance} = \frac{|(0 + 2 + 0) + 4|}{\sqrt{10}}
Distance=610=3105\text{Distance} = \frac{6}{\sqrt{10}} = \frac{3\sqrt{10}}{5}

答案

3105\frac{3\sqrt{10}}{5}

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