Solve produces 'set(a)' answer

[wiyosaya]

I am using Mathcad 13, and I know I have seen this before. I have searched the forums for the answer, however, I am unable to find it.

When using "Solve" to find the numeric value of a variable, in this case "a", Mathcad produces the answer "set(a)". Is there a way to force Mathcad to produce a numeric value?

Thanks in advance for your help.

[TomGutman]

Solve is a symbolic keyword. Using it implies an attempt to get a symbolic result. For numeric results you should use find or root.

Are you sure your equation even has a solution? Did you try plotting the expressions? The result of set(a) seems to be an idiosyncratic result from MC13, other versions simply fail to find a solution.
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� � � � Tom Gutman

[wiyosaya]

Thanks for the tips.

If using pi/3 as the angle, solve has no problem finding an answer.

As such, I would expect that using some multiple of pi/3 as the angle, in this case 2*pi/3, would also give an answer.

The equation is for the angle between two planes, and no matter the angle, I expect that it does have a solution.

Thanks again.

[TomGutman]

Plot it. It's pretty clear that while there is a solution with the angle of π/3, there is none with the angle of 2π/3. While planes in general can have any angle, your system implies constraints (no idea what the system is, or what any of the terms or variables in the equation are) and those constraints can easily exclude some ranges of angles.
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� � � � Tom Gutman

[rmix22-disabled]

Your equation has no solution (at least no real one) for angles between 0.637*pi and 1.363*pi.
Simply calculate the limit of the RHS for a going to infinity and take acos of that value.

As was already said here - plot the RHS of your equation and see, that it never can go as low as -0.5 (which you demanded by cos(2*pi/3).

Regards
RMix