Algebra · real student question

The graph shows the system y = 2/(x - 2) - 1 and y = 2|x - 2| - 4. Which set of points is the complete solution: (4,0); (0,0) and (4,0); (0,0), (0,-2) and (4,0); or (0,-2) and (4,0)?

Question

Which set of points represents the complete solution of the system

y=2x21,y=2x24 ?y=\frac{2}{x-2}-1,\qquad y=2|x-2|-4\ ?

(4,0)(0,0),(4,0)(0,0),(0,2),(4,0)(0,2),(4,0)(4,0)\qquad (0,0),(4,0)\qquad (0,0),(0,-2),(4,0)\qquad (0,-2),(4,0)

Step-by-step solution

  1. Substitute u = x - 2 to exploit the shared shift. Both curves are built around x=2x=2, so the substitution removes it from both:

    2u1=2u42u+3=2u,u0.\frac{2}{u}-1=2|u|-4\quad\Longrightarrow\quad \frac{2}{u}+3=2|u|,\qquad u\neq 0.

    The restriction u0u\neq 0 (that is, x2x\neq 2) comes from the vertical asymptote of the first curve.

  2. Split on the sign of u — case u > 0. Then u=u|u|=u, and multiplying through by u>0u>0 keeps the equation intact:

    2+3u=2u2  2u23u2=0  (2u+1)(u2)=0.2+3u=2u^{2}\ \Longrightarrow\ 2u^{2}-3u-2=0\ \Longrightarrow\ (2u+1)(u-2)=0.

    The roots are u=2u=2 and u=12u=-\tfrac12; only u=2u=2 satisfies u>0u>0, giving x=4x=4.

  3. Case u < 0. Then u=u|u|=-u, and multiplying by the negative uu flips nothing in an equation:

    2+3u=2u2  2u2+3u+2=0.2+3u=-2u^{2}\ \Longrightarrow\ 2u^{2}+3u+2=0.

    Its discriminant is 916=7<09-16=-7<0, so there is no real root and this case contributes nothing.

  4. Find the y-value of the surviving solution. At x=4x=4:

    y=2421=11=0,y=2424=44=0.y=\frac{2}{4-2}-1=1-1=0,\qquad y=2|4-2|-4=4-4=0.

    Both curves give y=0y=0, so (4,0)(4,0) is genuinely on both.

  5. Test the distractor points. At x=0x=0: the first curve gives y=221=2y=\tfrac{2}{-2}-1=-2, while the second gives y=2(2)4=0y=2(2)-4=0. So (0,0)(0,0) lies only on the absolute-value graph and (0,2)(0,-2) only on the rational graph — neither is a solution of the system. That is exactly the trap: a point on one curve is not a solution unless it is on both.

  6. State the complete solution. The system has exactly one solution, (4,0)(4,0).

Answer

{(4,0)}\{(4,0)\}

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