[Math] Find area of a triangle given the equation of sides

analytic geometrygeometry

Find the coordinates of the vertices of the triangle $ABC$ formed by the intersection of the lines $x + y = 0$, $x = 0$ and $y = x – 1$. Hence, find the area of the triangle.

I tried to sketch a graph but since the $y$ coordinate could be anywhere, I am having trouble working it out.

Best Answer

Following rschwieb's advice:

Note that

$\ \ \ \bullet$ $\color{darkgreen}{x=0}$ is the line coinciding with the $y$-axis.

$\ \ \ \bullet$ $\color{darkblue}{x+y=0}$ has standard form $y=-x$; so, its graph is the line of slope $-1$ passing through the origin.

$\ \ \ \bullet$ $\color{maroon}{y=x-1}$ is the equation of the line of slope $1$ with $y$-intercept $(0,-1)$.

The graphs of these lines are shown below:

enter image description here

The triangle in question is shaded light green (on my display) above. To find the coordinates of the vertices of this triangle, you have to solve systems of equations.

Apparently there is a vertex around the point $(1/2,-1/2)$. Let's check that. Here, we need to find the intersection of the lines $\color{darkblue}{x+y=0}$ and $\color{maroon}{y=x-1}$. I'll leave it to you to solve this system. It indeed has the solution $(1/2,-1/2)$.

Once you have the coordinates of all three vertices, computing the area of the triangle should be easy. For instance, you can take the base to be $1$ (along the $y$-axis) and the corresponding height to be $1/2$.

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