Algebra · real student question

Describe and graph the region of points satisfying both x + y <= 6 and x >= 2.

Question

Describe the set of points (x,y)(x,y) satisfying both

x+y6andx2x+y\le 6\qquad\text{and}\qquad x\ge 2

Step-by-step solution

  1. Treat the two inequalities as one system. A point belongs to the solution only if it satisfies both, so the answer is the intersection of two half-planes — not their union:

    {x+y6x2\begin{cases}x+y\le 6\\ x\ge 2\end{cases}

  2. Put the first inequality in slope-intercept form. Subtracting xx from both sides:

    y6xy\le 6-x

    The boundary y=6xy=6-x is a line of slope 1-1 through (0,6)(0,6) and (6,0)(6,0), drawn solid because of the \le. Shade everything on or below it.

  3. Interpret the second inequality. x2x\ge 2 is a vertical boundary line x=2x=2, again solid, with everything on or to the right of it shaded. Note that no yy appears, so this restriction is the same at every height.

  4. Locate the corner. The two boundaries meet where x=2x=2 and y=6xy=6-x, i.e. at

    (2,4)(2,\,4)

    This is the only vertex of the region; it belongs to the solution because both boundaries are included.

  5. Check the shape is unbounded. Starting at (2,4)(2,4) the region opens downward and to the right without limit: for instance (100,1000)(100,-1000) satisfies x2x\ge 2 and x+y=9006x+y=-900\le 6. So the region is an infinite wedge, not a triangle — there is no third boundary to close it off.

  6. State the region. Test (3,0)(3,0): 323\ge 2 \checkmark and 3+0=363+0=3\le 6 \checkmark, so it is inside. The solution set is

    {(x,y):x2 and y6x}\{(x,y): x\ge 2\ \text{and}\ y\le 6-x\}

Answer

{(x,y)  :  x2, y6x}— an unbounded wedge with corner (2,4)\{(x,y)\;:\;x\ge 2,\ y\le 6-x\}\quad\text{— an unbounded wedge with corner }(2,4)

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