Determining the type of triangle given 3 lines

Question

Determine the type of triangle that is formed by the lines x+y=11, x-y=1 and x-3y=3. Justify your decision?

Answer 1

For x + y = 11, y = -x + 11. So, the slope is -1.

For x - y = 1, y = x - 1. So, the slope is 1.

The lines x+y = 11 and x-y = 1 are perpendicular, since the slopes are negative reciprocals.

So, the triangle is a right triangle.