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Homework Help: Sin+Cos Question makes no sense to me

  1. May 15, 2007 #1
    Using a graph format in excel, plot the following function;
    Y=2SinӨ + 3CosӨ

    for values of Ө between 0 and 90???. From the graph find;
    a.the maximum amplitude of the function.
    2.A value of Ө which satisfies the equation
    2SinӨ + 3CosӨ = 3.5

    As evidence, you are required to show what formula you have used to produce the graph.

    ok well.. i put 2 in sin, and 3 in cos
    and did them added together to give a value, and tryed to make a graph..
    i dont get the question, or what im ment to do, or anything. please could someone help explain or give asmuch help as possible.

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  3. May 15, 2007 #2


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    Why must you use Excel? Are you permitted to use any graphing software program? If Excel contains trigonometric functions, you might be able to use
    something like =2*sin(x)+3*cos(x) for the "formula".
  4. May 15, 2007 #3


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    Staff: Mentor

    You need to make your first column the theta argument to the function. Put 0 in the first cell (call it A1), and in the second cell A2, put =A1+(90/100). Then click drag the second cell down to row 91, and use Ctrl-D to fill down that equation. That will give you a column for theta that varies from 0 to 90 in 1% increments.

    Then the second column should be your Y formula, with the arguments for the sin() and cos() being the cell just to the left. Does that make sense? So you end up with two columns, theta and Y(theta) = 2sin(theta)+3cos(theta). Then use the chart wizard or whatever to make an x-y scatter chart that plots Y(theta), with theta as the horizontal axis, and Y(theta) as the vertical axis.

    Does that get you back on track?
  5. May 15, 2007 #4


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    Staff: Mentor

    BTW, it would be good to double-check the default in Excel to see if the argument to sin() and cos() is assumed to be in degrees, like we are using so far in this thread. If the default is assumed to be radians, then you'll need to include the conversion in your formula for Y(theta).
  6. May 16, 2007 #5
    need to get the answer in degree's
    but excels degree's function seams to not be working as it should. also im unable to determin if its ment to be fluctuating up and down or not.
    any idea's?

    erm it goes up at 0.9 values, so highest value is 81.. so its not 1% increasments, please explain

    -also current problem.. im finding it impossible to convert it to degree's. me and others have tryed converting it using the dregee's function in excel, and radius*(180/Pi)
    but dosnt give the answer im suppose to get.


    there is an attempt of a table of what it is ment to look like ive been told.
    if dont understand table please ask and i will try and explain, but tbh i dont understand how they got those answers.
    Last edited: May 16, 2007
  7. May 16, 2007 #6
    Excel expects trig arguments in radians. To use degress, convert the degrees to radians with Excel's RADIANS() function ,eg SIN(RADIANS(a1))
  8. May 16, 2007 #7


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    Ah, good point. I should have said to fill down to 101, to get all 100 steps at 1% increments.
  9. May 16, 2007 #8
    ThoMcCloskey please read what you put compared to what i put.

    ok how will converting degree's which i need it to be in.. to a radius, which im guessing it is as defult at the moment. going to help?
    please read what i put.

  10. May 16, 2007 #9
    Well, I guess I don't understand what you mean or want. I was trying to suggest how to get the proper values for [tex]sin(\theta)[/tex] and [tex]cos(\theta)[/tex] when [tex]\theta[/tex] is expressed in degrees using Excel.

    What are you trying to convert to degrees?

    To plot the graph in Excel, simply construct the column of theta values (in degrees) as berkeman suggested. Then, construct another column with the expression you posted (Y) using the theta column and the radians conversion function (as I tried to suggest) so that you get the correct values of Y.

    Now plot the values of the expression (Y) against the values of theta (in degrees) and read the graph relative to theta (in degrees) to get your answers.

    P.S. - I don't understand your table.
  11. May 17, 2007 #10


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    Staff: Mentor

    College -- Did Theo's help solve your problem? Theo's advice was pretty clear.
  12. May 22, 2007 #11
    my mates been able to work out a function to work out the degree's, which is as follows
    im hopeing is right and im going to try implment it now =)

    using excels radians function degree's function didnt seam to work.

    btw im now 100% confused about radius and degree's and no idea what im doing =) just woke up, but guna have a go.
    p.s. sorry for seaming rude eirler

    ive had ago, and this is what ive got
    http://www.freewebs.com/rage_extreme/Sin%20Cos.xls [Broken]
    you reckon this is correct?
    Last edited by a moderator: May 2, 2017
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