error using solve function

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studentU
studentU 2015-5-13
评论: Torsten 2015-5-22
hi,
I use the function solve to determine a variable as follows:
x = solve (cos (x) * sin (y) * cos (z) + sin (Z) = 1 ',' x ')
then I evaluated x in terms of y and z, when I replace x by the value finding the equation is not = 1!!
how can i resolve it?

回答(1 个)

Walter Roberson
Walter Roberson 2015-5-13
Are x and y and z symbolic at the time you call solve? Or are y and z numeric?
If they are symbolic and you later substitute in numeric y and z to the solve() response get a numeric x, and then substitute the numeric x, y, and z into the cos (x) * sin (y) * cos (z) + sin (Z) formula, then you could get something other than 1 due to round-off error.
Also, if your y and z are square matrices of numeric values, then cos (x) * sin (y) * cos (z) would succeed but what not be what you want: you would need to evaluate
cos(x) .* sin(y) .* cos(z)
Notice the ".*" instead of "*" if you are working with vectors or matrices.
  4 个评论
studentU
studentU 2015-5-13
编辑:studentU 2015-5-13
I used simplify () but the result is always diferrent 1,
However, how to limit the interval of solution; for
x = solve ('cos (x) * sin (y) * cos (z) + sin (Z) = 1 ',' x ')
i want x beteween 0° and 90° for exmple,
thank's lot,
Torsten
Torsten 2015-5-22
I'd be interested what you get for x after applying "solve".
Best wishes
Torsten.

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