Carcass wrote:
A triangle ABC drawn on the coordinate plane has one side determined by points A = (1,3) and B = (5,6). The slope of the line BC is −3, while the slope of the line AC = 0. The coordinates of point C are
A (4,6)
B (1,18)
C (3,7)
D (6,3)
E (1,6)
If the slope of line AC is 0, we know immediately that the y-coordinate of point C is 3. Why is that true? Because if the slope is zero, the equation of the line in which both points lie is an horizontal line that passes through the point y=3, whose value of y does not change when X changes. Therefore, both points share the same y-coordinate.
Due to the fact that we already figured out the value of y, we now have find the x-coordinate of point C. Using the slope euation, we know that:
\(\frac{(y_{2} - y_{1})}{(x_{2}-x_{1})} = -3\)
\(\frac{(B_{y} - C_{y})}{(B_{x}-C_{x})} = -3\)
\(\frac{(6 - 3)}{(5-C_{x})} = -3\)
Doing some algebra:
\(C_{x} = 6. \)
Therefore, point C is (6,3), being Option D the correct answer.