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Question

Points P and Q have position vectors p = 2i − j - 3k and q = i + 4j - k. (a) Find the position vector of the midpoint M of [PQ]. (b) Point R lies on the line (PQ) such that QR = QM. Find the coordinates of R if R and M are distinct points.

Solution

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a)

Given that PP and QQ have position vectors p=2ij3kp=2i-j-3k and q=i+4jkq=i+4j-k

Let m=xi+yj+zkm=xi+yj+zk represents the position vector of MM the midpoint of [PQ]\left[PQ\right]

PQ=qp=i+4jk(2ij3k)=i+5j+2k\overrightarrow {PQ}=q-p=i+4j-k-\left(2i-j-3k\right)=-i+5j+2k

Since MM is the midpoint of [PQ]\left[PQ\right], then

PM=12PQ=12i+52j+k\overrightarrow {PM}=\frac{1}{2}\overrightarrow {PQ}=-\frac{1}{2}i+\frac{5}{2}j+k

OM=OP+PM\overrightarrow {OM}=\overrightarrow {OP}+\overrightarrow {PM}

xi+yj+zk=2ij3k12i+52j+k=32i32j2kxi+yj+zk=2i-j-3k-\frac{1}{2}i+\frac{5}{2}j+k=\frac{3}{2}i-\frac{3}{2}j-2k

the position vector of MM the midpoint of [PQ]\left[PQ\right] is

m=32i32j2km=\frac{3}{2}i-\frac{3}{2}j-2k

b)

Given that the point RR lies on the line (PQ)\left(PQ\right) such that

QR=QMQR=QM

Let r=xi+yj+zkr=xi+yj+zk represent the position vector of the point RR

OR=OP+PR=OP+32PQ\overrightarrow {OR}=\overrightarrow {OP}+\overrightarrow {PR}=\overrightarrow {OP}+\frac{3}{2}\overrightarrow {PQ}

xi+yj+zk=2ij3k+32(i+5j+2k)xi+yj+zk=2i-j-3k+\frac{3}{2}\left(-i+5j+2k\right)

=2ij3k+(32i+152j+3k)=12i+132j=2i-j-3k+\left(-\frac{3}{2}i+\frac{15}{2}j+3k\right)=\frac{1}{2}i+\frac{13}{2}j

Then

R(12,132,0)R\left(\frac{1}{2},\frac{13}{2},0\right)

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