Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Trigonometry homework help question

  1. May 21, 2004 #1
    in triangle PQR, PQ = a, QR = a + d, RP = a + 2d, and angle PQR is more than 120 degree., where a and d are positive constants. show that

    2a/3 < d < a.

    i tried to think, but i really dont know how to start to solve this question. hope for some clue.

    thank you
  2. jcsd
  3. May 21, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    Start with a picture, even if it doesn't help in the end, it's nice to have something to look at, you never know how it might inspire you. Now, we want to show:
    1) d < a
    2) d > 2a/3

    The first part is easy. Assume [tex]d \geq a[/tex]. Let [tex]d = a + \epsilon[/tex], where [tex]\epsilon \geq 0[/tex], [tex]\epsilon \in \lR[/tex].

    [tex]PQ + QR\ = 3a + \epsilon[/tex]
    [tex]PR = 3a + 2\epsilon[/tex]


    [tex]PR \geq PQ + QR[/tex]. Now, it should be obvious to you (especially if you try to draw it out with a ruler) that such a triangle is impossible. Our contradiction (that our triangle forms an impossible triangle) implies that our assumption is wrong (the assumption that [tex]d \geq a[/tex]), therefore, d is indeed less than a.

    2) Now, we know that the [tex]\angle PQR = q[/tex] satisfies:

    [tex]q > 120^o[/tex]


    [tex]\cos q < -0.5[/tex]

    Now, set up an equation using the Cosine Law:

    [tex](a + 2d)^2 = a^2 + (a + d)^2 - 2a(a + d)\cos q[/tex]
    [tex]\frac{a^2 - 2ad - 3d^2}{2a(a + d)} = \cos q[/tex]
    [tex]\frac{a^2 - 2ad - 3d^2}{2a(a + d)} < -0.5[/tex]
    [tex]a^2 - 2ad - 3d^2 < -a^2 - ad[/tex]
    [tex]2a^2 - ad - 3d^2 < 0[/tex]
    [tex](2a - 3d)(a + d) < 0[/tex]

    Therefore, either:

    [tex]2a - 3d < 0[/tex] AND [tex]a + d > 0[/tex], OR
    [tex]2a - 3d > 0[/tex] AND [tex]a + d < 0[/tex]

    It should be clearly impossible for "a + d < 0" to be true, therefore, we must meet the first set of conditions. Looking at "2a - 3d < 0", we get:

    [tex]2a < 3d[/tex]
    [tex]d > 2a/3[/tex]

    This proves 2). Finally, we have it that 2a/3 < d < a. Q.E.D.
  4. May 29, 2004 #3
    Thank You!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook