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A and C are the points (1, 3, -2) and (4, -3, 4) respectively - Scottish Highers Maths - Question 11 - 2016

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Question 11

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A and C are the points (1, 3, -2) and (4, -3, 4) respectively. Point B divides AC in the ratio 1 : 2. Find the coordinates of B. $ ext{Vector } ightarrow{AC}$ is a... show full transcript

Worked Solution & Example Answer:A and C are the points (1, 3, -2) and (4, -3, 4) respectively - Scottish Highers Maths - Question 11 - 2016

Step 1

Find the coordinates of B.

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Answer

To find the coordinates of point B that divides the line segment AC in the ratio 1:2, we can use the section formula.

Let A be represented by the coordinates (1, 3, -2) and C by (4, -3, 4). The section formula states that the coordinates of point B, which divides the line segment joining points A(x1, y1, z1) and C(x2, y2, z2) in the ratio m:n are given by:

B(x,y,z)=(mx2+nx1m+n,my2+ny1m+n,mz2+nz1m+n)B(x, y, z) = \left( \frac{mx_2 + nx_1}{m+n}, \frac{my_2 + ny_1}{m+n}, \frac{mz_2 + nz_1}{m+n} \right)

In our case, m = 1 and n = 2:

  1. Calculate x-coordinate of B: x=14+211+2=4+23=63=2x = \frac{1 \cdot 4 + 2 \cdot 1}{1 + 2} = \frac{4 + 2}{3} = \frac{6}{3} = 2

  2. Calculate y-coordinate of B: y=1(3)+231+2=3+63=33=1y = \frac{1 \cdot (-3) + 2 \cdot 3}{1 + 2} = \frac{-3 + 6}{3} = \frac{3}{3} = 1

  3. Calculate z-coordinate of B: z=14+2(2)1+2=443=03=0z = \frac{1 \cdot 4 + 2 \cdot (-2)}{1 + 2} = \frac{4 - 4}{3} = \frac{0}{3} = 0

Thus, the coordinates of point B are (2, 1, 0).

Step 2

Determine the value of k.

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Answer

To find the value of k such that the vector ightarrowAC ightarrow{AC} has a magnitude of 1, we first calculate the vector ightarrowAC ightarrow{AC}:

AC=CA=(4,3,4)(1,3,2)=(41,33,4(2))=(3,6,6)\rightarrow{AC} = C - A = (4, -3, 4) - (1, 3, -2) = (4 - 1, -3 - 3, 4 - (-2)) = (3, -6, 6)

Next, we find the magnitude of the vector ightarrowAC ightarrow{AC}:

AC=32+(6)2+62=9+36+36=81=9|\rightarrow{AC}| = \sqrt{3^2 + (-6)^2 + 6^2} = \sqrt{9 + 36 + 36} = \sqrt{81} = 9

We know that the magnitude must equal 1, so we can equate:

kAC=1k |\rightarrow{AC}| = 1

Substituting the magnitude we found:

k9=1k \cdot 9 = 1

Therefore:

k=19k = \frac{1}{9}

As required, k is greater than 0.

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