1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Word problem functions

  1. Jul 18, 2011 #1
    1. The problem statement, all variables and given/known data

    Sloping roof : The diagram shows a set of coordinate axes superimposed on the cross section of a sloping roof of height h metres and width w metres. The equation of one half of the cross section is

    y=-0.7x + 1.9

    a) What is the equation of the other half of the cross section? 0.7x + 1.9
    b) What is the height of the roof h? 1.9
    c) What is the width of the roof, w, to the nearest tenth of a metre?
    d) State the domain/range of the function that models each half of the roof.

    How do i do part c and d?

    2. Relevant equations

    3. The attempt at a solution

    No idea where to even begin
  2. jcsd
  3. Jul 18, 2011 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi Nelo! :smile:

    For c), what is the value of y at the two ends of the roof? And so what is the value of x ?

    For d), just draw a graph, using the answers to a) b) and c). :wink:
  4. Jul 18, 2011 #3


    User Avatar

    Staff: Mentor

    On c) you have two triangles that form the left and right halves of the roof. Use trig to calculate the lengths of the bases of those triangles.

    On d), do you know what the domain and range of a function are?
  5. Jul 18, 2011 #4


    User Avatar

    Staff: Mentor

    Dang it! TT strikes again! :biggrin:
  6. Jul 18, 2011 #5


    User Avatar
    Homework Helper

    Find the x-intercepts of both equations. Then find the distance between the x-intercepts.
    You should know the domain and range for lines. However, you are only interested in the line segments that form the roof. The x-intercept and the height (y-intercept) should give you a clue.

    EDIT: Beaten to it by TWO! :biggrin:
  7. Jul 18, 2011 #6
    I solved the x intercepts as plusminus 2.714 x.x
  8. Jul 18, 2011 #7
    The x intercepts are plusminus 2.714, what do you mean find the distance between? like, 2.7 + 2.7 = 5.4 , so the width is 5.4m ?

    for domain and range,

    I just graph -0.6x + 1.9 (a line with a vertical compression of 0.6?) and reflected?

    And the line 0.6x+1.9 being the f(x) and the top being a reflection on x axis?
  9. Jul 18, 2011 #8


    User Avatar

    Staff: Mentor

    Looks reasonable.
  10. Jul 18, 2011 #9
    0.7 is 7/10 as a fraction, do i rise 7 run 10? how do i graph that x.x

    also, does that mean that one equation taht represents height is this:

    y= -0.7x + 1.9

    and the other eq, that rep width is this?

    y= 0.7x + 2.7 ?
    Last edited: Jul 18, 2011
  11. Jul 18, 2011 #10
    ?? anyone?
  12. Jul 18, 2011 #11
    You have the slope right. As far as graphing goes, just treat it like any other line. Also, keep in mind 3.5/5 = 7/10, so for every five units you travel right, you should travel 3.5 units up.
    If that was the equation, the roof would be taller than 1.9 m if x was less than zero, correct? The height function should look like the roof itself, so what kind of function 'deflects' like the roof?
    x=0 corresponds to the peak of the roof, right? If so, do you think that the width at the peak is 2.7 m?
  13. Jul 18, 2011 #12
    *sigh* I have no idea, I just need to memorize how to do these by tommorow I usually just follow the script, so far I still dont get what the equations are so i can graph them
  14. Jul 18, 2011 #13


    User Avatar

    Staff: Mentor

    The equations are for two lines, using the form y(x) = mx + b. That looks familiar, right? What is the "m" term? What is the "b" term?

    Just plot the two lines that you have specified:

    y(x) = -0.7x + 1.9

    y(x) = 0.7x + 1.9

    Where do these two lines cross? Where do they intersect the y axis? Where do they intersect the x axis?
  15. Jul 18, 2011 #14
    um, the lines both cross at 1.9 on the y axis and cross at -2.5 on the x axis for the positive root.... and at 2.5 for the reflected root
  16. Jul 18, 2011 #15
    For the roof, the lines only make sense between were they intersect each other and where the intersect the x-axis. So, what x values 'make sense' for the lines?
  17. Jul 18, 2011 #16
    If they only make sense at the value they intersect at, then they are both going to have 1.9 and ones going to be -2.5 and one is going to be 2.5 , right?
  18. Jul 18, 2011 #17
    Not quite, but you have the right idea. The roof starts out at (-2.5, 0) and keeps going up until it hits the point (0, 1.9). So, for the equation that represents the left half of the roof (when it slopes upward), the only valid x values are between the x values of those two points. Does that make sense? (I'm not sure if I did a good job explaining it).

    [EDIT] Also, I think it should be -2.7 instead of 2.5, since 1.9/0.7 = 2.7
  19. Jul 18, 2011 #18
    right... the roof starts and ends.... the values between those points are only valid, like the triangle that is formed. how dose that help me though
  20. Jul 18, 2011 #19
    Well, let's keep thinking about the left side of the roof for now. The only valid x are -2.7<=x<=0. That's the domain, since the domain is the only x values a function makes sense on, so to speak. By the same token, the range is the only y values a function makes sense on. Can you see what the range is (what y values make sense for a roof?)
  21. Jul 18, 2011 #20
    Y values of roof are 0<==y==2
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook