With reference to the origin O, the point A has position vector −i−3j+3k-\mathbf{i} - 3\mathbf{j} + 3\mathbf{k}. The plane π\pi and the line ll have equations r=(421)+λ(−132)+μ(23−4)\mathbf{r} = \begin{pmatrix}4 \\ 2 \\ 1\end{pmatrix} + \lambda \begin{pmatrix}-1 \\ 3 \\ 2\end{pmatrix} + \mu \begin{pmatrix}2 \\ 3 \\ -4\end{pmatrix} and r=(211)+β(3t−2),\mathbf{r} = \begin{pmatrix}2 \\ 1 \\ 1\end{pmatrix} + \beta \begin{pmatrix}3 \\ t \\ -2\end{pmatrix}, respectively, where λ\lambda, μ\mu and β\beta are parameters and tt is a real value.

  1. (a)

    It is given that ll and π\pi intersect at a point.

    1. (i)

      Find the range of values of tt.

    2. (ii)

      Find, in terms of tt, the coordinates of the point of intersection, BB, of ll and π\pi.

  2. (b)

    It is given that AA lies on ll.

    1. (i)

      Show that t=4t = 4.

    2. (ii)

      Find the position vector of the foot of perpendicular, FF, of AA onto π\pi.

    3. (iii)

      Find the cartesian equation of another line mm through AA, that lies on the plane containing AA, BB and FF, and mm makes the same angle with π\pi as ll makes with π\pi.

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Worked solution

10(a)(i)

1. Find the normal vector and Cartesian equation of the plane $\pi$.

The normal vector to the plane π\pi is the cross product of its direction vectors:
n=(−132)×(23−4)=((3)(−4)−(2)(3)(2)(2)−(−1)(−4)(−1)(3)−(3)(2))=(−180−9)\mathbf{n} = \begin{pmatrix}-1 \\ 3 \\ 2\end{pmatrix} \times \begin{pmatrix}2 \\ 3 \\ -4\end{pmatrix} = \begin{pmatrix} (3)(-4) - (2)(3) \\ (2)(2) - (-1)(-4) \\ (-1)(3) - (3)(2) \end{pmatrix} = \begin{pmatrix} -18 \\ 0 \\ -9 \end{pmatrix}
Taking a simpler parallel normal vector n′=(201)\mathbf{n}' = \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix}, the Cartesian equation of the plane π\pi is:
r⋅(201)=(421)⋅(201)=8+0+1=9\mathbf{r} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = \begin{pmatrix} 4 \\ 2 \\ 1 \end{pmatrix} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = 8 + 0 + 1 = 9

2. Substitute the equation of line $l$ into the plane equation to find the intersection condition.

Substitute the equation of ll, r=(211)+β(3t−2)\mathbf{r} = \begin{pmatrix} 2 \\ 1 \\ 1 \end{pmatrix} + \beta \begin{pmatrix} 3 \\ t \\ -2 \end{pmatrix}, into the plane equation:
[(211)+β(3t−2)]⋅(201)=9\left[ \begin{pmatrix} 2 \\ 1 \\ 1 \end{pmatrix} + \beta \begin{pmatrix} 3 \\ t \\ -2 \end{pmatrix} \right] \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = 9
(4+0+1)+β(6+0−2)=9(4 + 0 + 1) + \beta (6 + 0 - 2) = 9
5+4β=95 + 4\beta = 9
Since tt is eliminated and we obtain 4β=44\beta = 4 (which yields a unique solution β=1\beta = 1), a unique point of intersection exists regardless of the value of tt. Thus, tt can be any real number.

Answer

t∈Rt \in \mathbb{R}

10(a)(ii)

1. Find the scalar product equation of the plane $\pi$ by calculating its normal vector.

The normal vector to the plane π\pi is the cross product of its two direction vectors:
n=(−132)×(23−4)=((3)(−4)−(2)(3)(2)(2)−(−1)(−4)(−1)(3)−(3)(2))=(−180−9)=−9(201)\mathbf{n} = \begin{pmatrix}-1 \\ 3 \\ 2\end{pmatrix} \times \begin{pmatrix}2 \\ 3 \\ -4\end{pmatrix} = \begin{pmatrix} (3)(-4) - (2)(3) \\ (2)(2) - (-1)(-4) \\ (-1)(3) - (3)(2) \end{pmatrix} = \begin{pmatrix} -18 \\ 0 \\ -9 \end{pmatrix} = -9\begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix}
Taking the normal vector as n=(201)\mathbf{n} = \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix}, the scalar product equation of π\pi is:
r⋅(201)=(421)⋅(201)\mathbf{r} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = \begin{pmatrix} 4 \\ 2 \\ 1 \end{pmatrix} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix}
r⋅(201)=8+0+1=9\mathbf{r} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = 8 + 0 + 1 = 9

2. Substitute the equation of line $l$ into the plane equation to find the intersection point $B$.

Substitute the vector equation of line ll into the scalar product equation of π\pi:
[(211)+β(3t−2)]⋅(201)=9\left[ \begin{pmatrix} 2 \\ 1 \\ 1 \end{pmatrix} + \beta \begin{pmatrix} 3 \\ t \\ -2 \end{pmatrix} \right] \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = 9
(2+3β)(2)+(1+tβ)(0)+(1−2β)(1)=9(2 + 3\beta)(2) + (1 + t\beta)(0) + (1 - 2\beta)(1) = 9
4+6β+1−2β=94 + 6\beta + 1 - 2\beta = 9
5+4β=95 + 4\beta = 9
4β=4  ⟹  β=14\beta = 4 \implies \beta = 1

Substitute β=1\beta = 1 back into the equation of line ll to find the position vector of BB:
OB⃗=(211)+(1)(3t−2)=(51+t−1)\vec{OB} = \begin{pmatrix} 2 \\ 1 \\ 1 \end{pmatrix} + (1)\begin{pmatrix} 3 \\ t \\ -2 \end{pmatrix} = \begin{pmatrix} 5 \\ 1+t \\ -1 \end{pmatrix}
The coordinates of BB are (5,1+t,−1)(5, 1+t, -1).

Answer

(5,1+t,−1)(5, 1+t, -1)

10(b)(i)

1. Equate the position vector of point $A$ to the equation of line $l$.

(−1−33)=(211)+β(3t−2)\begin{pmatrix}-1 \\ -3 \\ 3\end{pmatrix} = \begin{pmatrix}2 \\ 1 \\ 1\end{pmatrix} + \beta \begin{pmatrix}3 \\ t \\ -2\end{pmatrix}

2. Solve for $\beta$ using the $x$-component and substitute into the $y$-component to find $t$.

From the xx-components:
−1=2+3β  ⟹  3β=−3  ⟹  β=−1-1 = 2 + 3\beta \implies 3\beta = -3 \implies \beta = -1

(Checking with zz-components: 3=1−2(−1)=33 = 1 - 2(-1) = 3, consistent.)

From the yy-components:
−3=1+βt-3 = 1 + \beta t
Substitute β=−1\beta = -1:
−3=1−t  ⟹  t=4 (shown)-3 = 1 - t \implies t = 4 \text{ (shown)}

Answer

Shown

10(b)(ii)

1. Find a normal vector to the plane $\pi$.

The normal vector n\mathbf{n} can be found using the cross product of the two direction vectors of the plane π\pi:

n=(−132)×(23−4)\mathbf{n} = \begin{pmatrix} -1 \\ 3 \\ 2 \end{pmatrix} \times \begin{pmatrix} 2 \\ 3 \\ -4 \end{pmatrix}

n=((3)(−4)−(2)(3)(2)(2)−(−1)(−4)(−1)(3)−(3)(2))=(−180−9)\mathbf{n} = \begin{pmatrix} (3)(-4) - (2)(3) \\ (2)(2) - (-1)(-4) \\ (-1)(3) - (3)(2) \end{pmatrix} = \begin{pmatrix} -18 \\ 0 \\ -9 \end{pmatrix}

Taking a simpler parallel vector by dividing by −9-9, we use n=(201)\mathbf{n} = \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix}.

2. Formulate the scalar product equation of the plane and the vector equation of the normal line through $A$.

The equation of plane π\pi is r⋅(201)=(421)⋅(201)=8+0+1=9\mathbf{r} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = \begin{pmatrix} 4 \\ 2 \\ 1 \end{pmatrix} \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = 8 + 0 + 1 = 9.

The normal line passing through AA perpendicular to π\pi has the equation:
lA:r=(−1−33)+α(201), where α∈Rl_A: \mathbf{r} = \begin{pmatrix} -1 \\ -3 \\ 3 \end{pmatrix} + \alpha \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix}, \text{ where } \alpha \in \mathbb{R}

3. Substitute the line equation into the plane equation to find the foot of the perpendicular, $F$.

Substitute lAl_A into the equation of π\pi:

[(−1−33)+α(201)]⋅(201)=9\left[ \begin{pmatrix} -1 \\ -3 \\ 3 \end{pmatrix} + \alpha \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} \right] \cdot \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = 9

(−2+0+3)+α(22+02+12)=9(-2 + 0 + 3) + \alpha(2^2 + 0^2 + 1^2) = 9

1+5α=9  ⟹  5α=8  ⟹  α=851 + 5\alpha = 9 \implies 5\alpha = 8 \implies \alpha = \frac{8}{5}

Substitute α=85\alpha = \frac{8}{5} back into the line equation to find the position vector of FF:

OF→=(−1−33)+85(201)=(−1+16/5−3+03+8/5)=(11/5−323/5)=15(11−1523)\overrightarrow{OF} = \begin{pmatrix} -1 \\ -3 \\ 3 \end{pmatrix} + \frac{8}{5} \begin{pmatrix} 2 \\ 0 \\ 1 \end{pmatrix} = \begin{pmatrix} -1 + 16/5 \\ -3 + 0 \\ 3 + 8/5 \end{pmatrix} = \begin{pmatrix} 11/5 \\ -3 \\ 23/5 \end{pmatrix} = \frac{1}{5}\begin{pmatrix} 11 \\ -15 \\ 23 \end{pmatrix}

Answer

15(11−1523)\frac{1}{5}\begin{pmatrix} 11 \\ -15 \\ 23 \end{pmatrix}

10(b)(iii)

1. Concisely recover the coordinates of intersection point $B$ and foot of perpendicular $F$ from earlier parts.

The normal to π\pi is n=(−132)×(23−4)=(−180−9)≡(201)\mathbf{n} = \begin{pmatrix}-1\\3\\2\end{pmatrix} \times \begin{pmatrix}2\\3\\-4\end{pmatrix} = \begin{pmatrix}-18\\0\\-9\end{pmatrix} \equiv \begin{pmatrix}2\\0\\1\end{pmatrix}, so π\pi has equation r⋅(201)=9\mathbf{r} \cdot \begin{pmatrix}2\\0\\1\end{pmatrix} = 9.
Substituting ll (with t=4t=4) into π\pi: (2+3β1+4β1−2β)⋅(201)=9  ⟹  4+6β+1−2β=9  ⟹  β=1\begin{pmatrix}2+3\beta\\1+4\beta\\1-2\beta\end{pmatrix} \cdot \begin{pmatrix}2\\0\\1\end{pmatrix} = 9 \implies 4 + 6\beta + 1 - 2\beta = 9 \implies \beta = 1.
Thus, BB has position vector OB⃗=(55−1)\vec{OB} = \begin{pmatrix}5\\5\\-1\end{pmatrix}.
The line AFAF passes through A(−1,−3,3)A(-1, -3, 3) and is parallel to n\mathbf{n}, so r=(−1−33)+α(201)\mathbf{r} = \begin{pmatrix}-1\\-3\\3\end{pmatrix} + \alpha \begin{pmatrix}2\\0\\1\end{pmatrix}.
Substituting into π\pi: 2(−1+2α)+(3+α)=9  ⟹  5α=8  ⟹  α=852(-1+2\alpha) + (3+\alpha) = 9 \implies 5\alpha = 8 \implies \alpha = \frac{8}{5}.
Thus, FF has position vector OF⃗=(11/5−323/5)\vec{OF} = \begin{pmatrix}11/5\\-3\\23/5\end{pmatrix}.

2. Use the geometric properties of the plane $ABF$ to determine the point $B'$ where line $m$ intersects $\pi$.

Since line mm lies on the plane containing AA, BB and FF, and makes the same angle with π\pi as ll, mm must intersect the line BFBF (the projection of ll on π\pi) at a point B′B'.
Because AF⊥πAF \perp \pi, △ABB′\triangle ABB' forms an isosceles triangle with altitude AFAF. Hence, FF is the midpoint of BB′BB'.
FB′⃗=BF⃗  ⟹  OB′⃗=2OF⃗−OB⃗\vec{FB'} = \vec{BF} \implies \vec{OB'} = 2\vec{OF} - \vec{OB}
OB′⃗=2(11/5−323/5)−(55−1)=(22/5−646/5)−(25/55−5/5)=(−3/5−1151/5)\vec{OB'} = 2 \begin{pmatrix} 11/5 \\ -3 \\ 23/5 \end{pmatrix} - \begin{pmatrix} 5 \\ 5 \\ -1 \end{pmatrix} = \begin{pmatrix} 22/5 \\ -6 \\ 46/5 \end{pmatrix} - \begin{pmatrix} 25/5 \\ 5 \\ -5/5 \end{pmatrix} = \begin{pmatrix} -3/5 \\ -11 \\ 51/5 \end{pmatrix}

3. Find the direction vector of $m$ and deduce its cartesian equation.

The direction vector of mm is given by AB′⃗\vec{AB'}:
AB′⃗=OB′⃗−OA⃗=(−3/5−1151/5)−(−1−33)=(2/5−836/5)\vec{AB'} = \vec{OB'} - \vec{OA} = \begin{pmatrix} -3/5 \\ -11 \\ 51/5 \end{pmatrix} - \begin{pmatrix} -1 \\ -3 \\ 3 \end{pmatrix} = \begin{pmatrix} 2/5 \\ -8 \\ 36/5 \end{pmatrix}
To simplify the direction vector, we can factor out a scalar multiplier. Multiplying by 52\frac{5}{2} gives an integer direction vector:
dm=(1−2018)\mathbf{d}_m = \begin{pmatrix} 1 \\ -20 \\ 18 \end{pmatrix}
Using point A(−1,−3,3)A(-1, -3, 3) and direction vector dm\mathbf{d}_m, the cartesian equation for mm is:
x+11=y+3−20=z−318\frac{x + 1}{1} = \frac{y + 3}{-20} = \frac{z - 3}{18}

Answer

x+11=y+3−20=z−318\frac{x + 1}{1} = \frac{y + 3}{-20} = \frac{z - 3}{18}

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