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Next code gives me 5.999999999999998 in Result, but right answer is 6.
Alpha = math:acos((4*4 + 5*5 - 3*3) / (2*4*5))
Area = 1/2 * 4 * 5 * math:sin(Alpha)
Is it possible to get 6?
You have run into a problem so common that it has its own web site, What Every Programmer Should Know About Floating-Point Arithmetic. The problem is due to the way floating-point arithmetic works in pretty much every CPU on the market that supports FP arithmetic; it is not specific to Erlang.
If regular floating point arithmetic does not give you the precision or accuracy you need, you can use an arbitrary precision arithmetic library instead of the built-in arithmetic. Perhaps the most well-known such library is GMP, but you’d have to wrap it in NIFs to use it from Erlang.
There is at least one pure-Erlang alternative, but I have no experience with it, so I cannot personally endorse it.