MHB Equation of Line Passing Through \(A(-5,-4)\) & \(B, C\) & \(D\)

  • Thread starter Thread starter sbhatnagar
  • Start date Start date
  • Tags Tags
    Line
AI Thread Summary
To find the equation of the line passing through point \(A(-5,-4)\) that intersects three given lines at points \(B\), \(C\), and \(D\), the gradient \(m\) of the line is established using the relationship \(\frac{y_i+4}{x_i+5}=m\) for each intersection point. The coordinates of points \(B\), \(C\), and \(D\) are determined by substituting \(x\) and \(y\) values into the equations of the respective lines. By expressing the coordinates in terms of \(m\) and applying the condition \(\left(\frac{15}{AB}\right)^2 + \left(\frac{10}{AC}\right)^2 = \left(\frac{6}{AD}\right) ^2\), the value of \(m\) can be calculated. Ultimately, this leads to the determination of the line's equation. The method outlined provides a systematic approach to solving the problem.
sbhatnagar
Messages
87
Reaction score
0
A line through \(A(-5,-4)\) meets the lines \(x+3y+2=0\), \(2x+y+4=0\) and \(x-y-5=0\) at the points \(B, \ C\) and \(D\) respectively. If

\[\left(\frac{15}{AB}\right)^2 + \left(\frac{10}{AC}\right)^2 = \left(\frac{6}{AD}\right) ^2 \]

, find the equation of the line.
 
Mathematics news on Phys.org
sbhatnagar said:
A line through \(A(-5,-4)\) meets the lines \(x+3y+2=0\), \(2x+y+4=0\) and \(x-y-5=0\) at the points \(B, \ C\) and \(D\) respectively. If

\[\left(\frac{15}{AB}\right)^2 + \left(\frac{10}{AC}\right)^2 = \left(\frac{6}{AD}\right) ^2 \]

, find the equation of the line.

Hi sbhatnagar, :)

I shall outline the method to solve this problem.

Let \(m\) be the gradient of the line through \(A(-5,-4),\,B(x_{1},y_{1}),\,C(x_{2},y_{2})\mbox{ and }D(x_{3},y_{3})\). Then,

\[\frac{y_i+4}{x_i+5}=m\mbox{ for each }i=1,\,2,\,3\]

Also since the points B, C and D are on the lines given by \(x+3y+2=0\), \(2x+y+4=0\) and \(x-y-5=0\) respectively, we have,

\[x_1+3y_1+2=0\]

\[2x_2+y_2+4=0\]

\[x_{3}-y_{3}-5=0\]

Using the above six equations we can find \(x_1,\,x_{2},\,x_{3},\,y_1,\,y_{2},\mbox{ and }y_{3}\) in terms of \(m\).

Finally using the given equation, \(\left(\frac{15}{AB}\right)^2 + \left(\frac{10}{AC}\right)^2 = \left(\frac{6}{AD}\right) ^2 \) the value of \(m\) can be found. Hence the equation of the line can be determined.

Kind Regards,
Sudharaka.
 
Last edited:
My solution can be found in the spoiler.

Let equation of line \(AC\) be

\[\frac{y+4}{\sin \theta}=\frac{x+5}{\cos \theta}=r\]

Let line \(AE\) make angle \(\theta\) with the \(x\)-axis and intersects \(x+3y+2=0\) at \(B\) at a distance \(r_1\) and line \(2x+y+4=0\) at \(C\) at a distance \(r_2\) and line \(x-y-5=0\) at \(D\) at a distance \(r_3\).
\[\therefore \ AB=r_1 , \ AC=r_2, \ AD=r_3\]

Putting \(x=r_1\cos \theta -5\) and \(y=r_1\sin \theta -4\) in \(x+3y+2 =0\) we get

\[\begin{aligned}
x+3y+2 &=0 \\
\Rightarrow r_1 \cos \theta -4 +3(r_1 \sin \theta -4)+2 &=0 \\
\Rightarrow r_1 &= \frac{-5-3(4)+2}{\cos \theta +3 \sin \theta} \\
\Rightarrow r_1 &= \frac{15}{\cos \theta +3 \sin \theta} \quad \cdots \text{(i)}

\end{aligned}\]

Similarly,

\[\begin{aligned}
r_2 &= \frac{10}{2\cos \theta + \sin \theta} \quad \cdots \text{(ii})\\
r_3 &= \frac{6}{\cos \theta - \sin \theta} \quad \cdots \text{(iii)}

\end{aligned}\]

But it is given that

\[\begin{aligned} \left(\frac{15}{AB}\right)^2 + \left(\frac{10}{AC}\right)^2 &= \left(\frac{6}{AD}\right)^2 \\ \Rightarrow \left(\frac{15}{r_1}\right)^2 + \left(\frac{10}{r_2}\right)^2 &= \left(\frac{6}{r_3}\right)^2 \\ \Rightarrow (\cos \theta + 3\sin \theta )^2+(2\cos \theta + \sin \theta)^2 &=(\cos \theta -\sin \theta)^2 \quad [\text{from equations (i), (ii) and (iii)}] \\ 4\cos^2 \theta + 6 \sin ^2 \theta +12 \sin \theta \cos \theta &=0 \\ (2\cos \theta + 3 \sin \theta )^2 &=0 \\ 2\cos \theta + 3 \sin \theta &=0 \\ \tan \theta &=-\frac{2}{3}\end{aligned}\]

On substituting this in the equation of \(AC\), we get

\[y+4=(x+5)\tan \theta \\ \Rightarrow y+4=-\frac{2}{3}(x+5) \\ \Rightarrow 2x+3y+22=0\]
 

Attachments

  • figure.png
    figure.png
    2 KB · Views: 100
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Thread 'Imaginary Pythagoras'
I posted this in the Lame Math thread, but it's got me thinking. Is there any validity to this? Or is it really just a mathematical trick? Naively, I see that i2 + plus 12 does equal zero2. But does this have a meaning? I know one can treat the imaginary number line as just another axis like the reals, but does that mean this does represent a triangle in the complex plane with a hypotenuse of length zero? Ibix offered a rendering of the diagram using what I assume is matrix* notation...

Similar threads

Replies
4
Views
1K
Replies
2
Views
2K
Replies
8
Views
2K
Replies
4
Views
11K
Replies
3
Views
2K
Replies
6
Views
2K
Replies
2
Views
5K
Replies
20
Views
2K
Replies
1
Views
2K
Back
Top