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elica85

  • 3 years ago

can anyone help with Matlab?

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  1. hali
    • 3 years ago
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    What problem are you facing in Matlab?

  2. elica85
    • 3 years ago
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    i have to write a program for this problem... determine the number of terms necessary to approximate cosx to 8 significant figures using the Maclaurin series approximation. use x=0.3pi \[cosx=1-(x ^{2}/2)+(x ^{4}/4!)-(x ^{6}/6!)+(x ^{8}/8!)...\] i don't know where to begin. I took Matlab 2 summers ago and the school and instructor was horrible and i'm not just saying this. the instructor stayed on the first 2 chapters (learning the basic functions) for half the semester and barely taught us how to write our own programs. i think she's fired.

  3. phi
    • 3 years ago
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    Here is matlab that should work % cos(x) = 1 - (x^2)/2! + (x^4)/4! -(x^6)/6!+(x^8)/8!... % let y= x*x % cos(x) = sum( (-y)^n/(2n)! ) format short x= 0.3*pi; y= x*x; for N= 1:6 n= 0:N; s1= [(-y).^n./factorial(2*n) ] mac= sum(s1); cx= cos(x); str= sprintf('%d terms. series: %12.10f cos(x): %12.10f\n %12.10f',... N, mac,cx, (cx-mac)); disp(str); end;

  4. experimentX
    • 3 years ago
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    differetiate cos(x) iteratively at x=0 y = cos (t) loop { y = diff( y , t); sub(y, 0) } find the coefficients ... use the usual method.

  5. elica85
    • 3 years ago
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    @phi thx, did you solve the problem to 10 sig figures?

  6. phi
    • 3 years ago
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    It prints out more than 8. I'll leave it to you to count the digits

  7. elica85
    • 3 years ago
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    ok, how do i format the number of digits? so far i was able to find the commands "short", "long", but not exact number of digits i want

  8. phi
    • 3 years ago
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    try running the program. the sprintf statement formats using the C style formatting %12.10f means use 12 places (includes decimal point), with 10 places to the right of the decimal

  9. elica85
    • 3 years ago
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    ok, i'll try thx

  10. phi
    • 3 years ago
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    Although be careful, because I can't count! where it says it is using n terms, it is really using n+1 terms, counting the initial 1

  11. elica85
    • 3 years ago
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    i'm actually really confused and writing everything out on paper to see where everything fits. why is y=x*x, if s1 suppose to equal the explicit form of the mac series, why is it (-y)^n instead of (-1)^n and where's x^(2n)? i'm comparing to the explicit form: \[cosx=\sum_{n=0}^{\infty} \frac{ (-1)^n }{ (2n)! } x ^{2n}\]

  12. phi
    • 3 years ago
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    you do know \( x^{2n} = (x^2)^n \), right? and \( (-1)^n \cdot (x^2)^n= (-x^2)^n \)

  13. elica85
    • 3 years ago
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    ok thx

  14. phi
    • 3 years ago
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    It just makes the code a little bit simpler (or maybe just a little bit more indecipherable).

  15. elica85
    • 3 years ago
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    ok, and back to the number of decimal places...how would changing the %12.10f change anything since it's after %?

  16. phi
    • 3 years ago
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    See above for the explanation. %12.0f would print out up to 11 digits plus the decimal point with 0 of the digits to the right of the point. It is easier to play with it.

  17. elica85
    • 3 years ago
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    it worked. thx again for the help

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