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The line l contains the points A(4, 5) and B(2, 0) - Leaving Cert Mathematics - Question 4 - 2016

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The line l contains the points A(4, 5) and B(2, 0). Find the equation of l. Give your answer in the form ax + by + c = 0 where a, b, and c ∈ ℤ. Draw the line k: x +... show full transcript

Worked Solution & Example Answer:The line l contains the points A(4, 5) and B(2, 0) - Leaving Cert Mathematics - Question 4 - 2016

Step 1

Find the equation of l.

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Answer

To find the equation of the line l that passes through the points A(4, 5) and B(2, 0), we first calculate the slope (m) using the formula:

m=y2y1x2x1=0524=52=52m = \frac{y_2 - y_1}{x_2 - x_1} = \frac{0 - 5}{2 - 4} = \frac{-5}{-2} = \frac{5}{2}

Next, we use the point-slope form of the equation of a line:

yy1=m(xx1)y - y_1 = m(x - x_1)

Substituting point A(4, 5):

y5=52(x4)y - 5 = \frac{5}{2}(x - 4)

Expanding and rearranging to the standard form:

y5=52x10y - 5 = \frac{5}{2}x - 10

2y10=5x202y - 10 = 5x - 20

5x2y+10=05x - 2y + 10 = 0

Thus, the equation of the line l is:

5x2y+10=05x - 2y + 10 = 0

Step 2

Draw the line k: x + 2y = 8.

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Answer

To draw the line k, we can rewrite it in slope-intercept form.

Starting from:

x+2y=8 x + 2y = 8

We isolate y:

2y=x+82y = -x + 8

y=12x+4y = -\frac{1}{2}x + 4

Now, we plot the y-intercept at (0, 4) and another point by letting x = 8:

y=12(8)+4=0y = -\frac{1}{2}(8) + 4 = 0

So, we plot the second point at (8, 0). Next, we connect these points to illustrate the line k.

Step 3

Use a graphic, numeric or algebraic method to find the co-ordinates of l ∩ k.

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Answer

To find the coordinates where lines l and k intersect, we can solve the equations:

  1. From the equation of line l: 5x2y+10=05x - 2y + 10 = 0 Rearranged: 2y=5x+102y = 5x + 10 y=52x+5y = \frac{5}{2}x + 5

  2. From the equation of line k: x+2y=8    2y=8x    y=412xx + 2y = 8 \implies 2y = 8 - x \implies y = 4 - \frac{1}{2}x

Set the two equations for y equal to each other:

52x+5=412x\frac{5}{2}x + 5 = 4 - \frac{1}{2}x

Solve for x:

52x+12x=45\frac{5}{2}x + \frac{1}{2}x = 4 - 5

3x=1    x=133x = -1 \implies x = -\frac{1}{3}

Substituting back to find y:

y=412(13)=4+16=246+16=256y = 4 - \frac{1}{2}\left(-\frac{1}{3}\right) = 4 + \frac{1}{6} = \frac{24}{6} + \frac{1}{6} = \frac{25}{6}

Thus, the coordinates of intersection l ∩ k are:

(13,256)\left(-\frac{1}{3}, \frac{25}{6}\right)

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