x < 6y - y²
Graph the boundary curve by replacing the < by =
x = 6y - y²
y² - 6y = -x
Complete the square on the left by adding (·(-3)² = 9
to both sides:
y² - 6y + 9 < -x + 9
Factor the left side
(y - 3)² = -x + 9
This is in the form of a parabola with
horizontal line of symmetry
(y - k)² = 4p(x - h)
(y - 3)² = -1(x - 9)
The vertex is (h,k) = (9,3) and it opens left since 4p = -1
p = which is negative:
You can get a few points in addition to the vertex, say
(5,5), (5,1), (0,6), (0,0), (-7,7), (-7,-1)
Now the question is whether to shade the inside portion or the outside
portion of the parabola. So we test a point that's not on the parabola,
say (3,4), which is inside the parabola, in the original inequality:
We substitute that into the original inequality:
x < 6y - y²
3 < 6(4) - (4)²
3 < 24 - 16
3 < 8
This is true, so since (3,4) is a solution and it is on the
inside of the parabola, all the points inside the parabola
are solutions.
Edwin