=================================
On the coordinate plane there are 3 points 𝐴(−1, 1), 𝐵(1, −3), and 𝐶(3, 4). What is the area of the triangle connecting the 3 points?
(A) 10
(B) 11
(C) 12
(D) 14
(E) 13
=================================We want to take the area. Area is 0.5 * base * height -- where height is the altitude of the triangle.
I want AB to be the base of my triangle. This means that we want the altitude dropped from C to be our height.
To simplify things, I'm going to move the triangle such that B sits on the origin; this moves the triangle -1 in the x-direction and +3 in the y direction.
A -> (-2, 4)
B -> (0, 0)
C -> (2, 7)
Let's check if we're lucky and it's a right triangle.
-> Slope of AB is -4 / 2 = -2
-> Slope of BC is 7 / 2 = 3.5
-> Nope!
So we have to tackle the altitude in two steps:
(1) Figure out the point D where the continuation of AB intersects a perpendicular line that runs through point C
(2) Take the distance between C and D
Because it runs through the origin, we can just write the equation of AB as y = -2x --> we know this describes our point D
We then can write the equation of CD.
--> The slope of CD is the negative reciprocal of the slope of AB
--> CD runs through C
So we get y - 7 = 0.5(x -2); we can plug in our equation from AB.
\(y-7 = 0.5(x-2)\)
\((-2x)-7 = 0.5(x-2)\)
\(-2x-7=0.5x-1\)
\(-6 = 2.5x\)
\(x = -6/2.5 = -12/5 = -2.4\)
We plug in again for y and get that y = 4.8.
Now we need the distance between C and D
--> C lies at (2, 7)
--> D lies at (-2.4, 4.8)
\(\sqrt{(2 - (-2.4))^2 + (7 - 4.8)^2}\)
\(=\sqrt{(4.4)^2 + (2.2)^2}\)
\(=\sqrt{19.36 + 4.84}\)
\(= \frac{1}{10} * \sqrt{1936 + 484}\)
\(= \frac{1}{10} * \sqrt{2420}\)
\(= \frac{{11 * 2} }{ 10} * \sqrt{5}\)
And we need the distance from A to B:
\(\sqrt{2^2 + 4^2}\)
\(=\sqrt{4 + 16}\)
\(=\sqrt{20}\)
\(= 2\sqrt{5}\)
Then we multiply the distance from A to B by the distance from C to D and then multiply that by 0.5
Area =
\(\frac{{11 * 2} }{ 10} * \sqrt{5} * 2\sqrt{5} * \frac{1}{2}\)
Area = 11