Hello, James!
Find the inequalities that define the interior of a triangle
with vertices: . First, graph the points . . . Code:
 C
 o (1,5)
 * *
 * *
* * B
* o (3,1)
* *
*+*
* *
A o 
(1,1)

Find the equation of line
. . Slope of
. . Equation: .
. . We want the region above this line: .
Find the equation of line
. . Slope of
. . Equation: .
. . We want the region below this line: .
Find the equation of line
. . Slope of
. . Equation: .
. . We want the region below this line: .
Therefore: .