THis is not a Qt Script problem, but a computer problem. There is a limit to the precision of numbers you can express in floating point numbers. That is what you are seeing. You should compare them using an epsilon.

Limited numerical precision of the floating point representation of numbers is the problem.

As i understude, you dont understunde me(maybe i am dont right, my english is bad) i say not about
e-16 in 1.2246467991473532e-16, i say about sin(pi) should be equal 0 as cos(pi/2)

As far as I understand the problem is that Math.PI does not express the pure PI, since Math.PI is limited by the precision of the computer floating numbers. I guess you will have the same result (or a similar one) using another math library.

Ruzik wrote:

As i understude, you dont understunde me(maybe i am dont right, my english is bad) i say not about e-16 in 1.2246467991473532e-16, i say about sin(pi) should be equal 0 as cos(pi/2)

I’m afraid they are right, mate. With floating point, you cannot really get exactly 0, save for a few cases.

Ruzik wrote:

As i understude, you dont understunde me(maybe i am dont right, my english is bad) i say not about e-16 in 1.2246467991473532e-16, i say about sin(pi) should be equal 0 as cos(pi/2)

Well, 6.123233995736766e-17 ≈ 0, as far as floating point numbers are concerned.

Clear, thank you for your help!

Also, the trigonometric functions are transcendental functions and so cannot be calculated exactly. All implementations use approximations that are good to some degree of accuracy for a range of inputs.

As Andre suggested, test it with an epsilon (small value) to see if it is in agreement with your criteria. i.e.

Obviously you can use different values for eps and test as needed in your particular cases. This is just a fact of life when working with floating point representations on machines with limited memory.