- #1
binglee
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Hi,
I know this one is stupid, but i am still confused. why e^(iy) = cosy + i siny?
thank you.
I know this one is stupid, but i am still confused. why e^(iy) = cosy + i siny?
thank you.
binglee said:Hi,
I know this one is stupid, but i am still confused. why e^(iy) = cosy + i siny?
thank you.
mathman said:It is not by definition. It is a theorem, most easily proven by matching power series.
No, it's a theorem, i.e. it has been proved.wofsy said:One absolutley can take this as a definition.
mathman said:The exponential, sine, and cosine functions have precise definitions. The usual definitions are as follows:
e=lim(n->oo) (1 + 1/n)sup[n][/sup]
sin=ratio of side opposiste to hypotenuse of right triangle.
cos=ratio of adjacent side to hypotenuse of right triangle.
Given these definitions, the powers series for these functions can be derived and the relationship proved. It is NOT a definition of anything!
Those are NOT the "usual definitions" and the power series for the functions cannot be derived from them. For one thing, those definitions do not define sin(x) for x greater than [itex]\pi/2[/itex] or less than 0.mathman said:The exponential, sine, and cosine functions have precise definitions. The usual definitions are as follows:
e=lim(n->oo) (1 + 1/n)sup[n][/sup]
sin=ratio of side opposiste to hypotenuse of right triangle.
cos=ratio of adjacent side to hypotenuse of right triangle.
Given these definitions, the powers series for these functions can be derived and the relationship proved. It is NOT a definition of anything!
binglee said:I know this one is stupid, but i am still confused. why e^(iy) = cosy + i siny?
zhentil said:This thread is funny. I thought that if four statements were equivalent and one was a definition, any of them could be taken as the definition.
The (...) sine, and cosine functions have precise definitions. The usual definitions are (...)
sin=ratio of side opposiste to hypotenuse of right triangle.
cos=ratio of adjacent side to hypotenuse of right triangle.
I am being stubborn only about one point, which Halls of Ivy already stated. The sine, cosine, and exponential functions can be defined in many ways. I happen to use the simplest, where the domains can be extended to all complex numbers by analytic continuation.Landau said:I believe that is not all he said. He also said to reject the fact that the complex exponential can be defined in terms of the real cosine and sine, and
Yes, that's true. And if you had said that at first, I would not have complained. But you gave a definition of e and said that was the definition of the function ex.mathman said:Once e is defined, ex can be defined just like any other power of a positive number, first by defining integer powers, then defining integer roots, then all rational powers, and finally all powers.
mathman said:However, the main point that I have stressing is that Euler's theorem is a theorem, and NOT a definition.
elibj123 said:There is always the ODE approach. notice that:
d(cos(x)+i*sin(x))/dx=(-sin(x)+i*cos(x))=i*(cos(x)+i*sin(x))
you got a function y(x) which satisfies:
y'=iy
y(0)=1
and the solution of that is
y=exp(ix)
An exponential function in complex plane is a function of the form e^x(cosy+isiny), where x is a real number and y is an imaginary number. This function is also known as the Euler's formula and can be represented as a point on the complex plane, with x as the real part and y as the imaginary part.
The complex exponential function, e^x(cosy+isiny), is closely related to the real exponential function, e^x. In fact, when y is equal to 0, the complex exponential function reduces to the real exponential function. This means that the real exponential function is a special case of the complex exponential function.
The complex exponential function is defined as e^x(cosy+isiny) because it is a natural extension of the real exponential function. This form allows for the representation of complex numbers on the complex plane and has many useful mathematical properties.
The cosine and sine terms in the definition of the complex exponential function represent the real and imaginary parts of the function, respectively. These terms are essential in defining the function on the complex plane and allow for the representation of complex numbers in polar form.
The complex exponential function has many applications in science, particularly in fields such as physics, engineering, and mathematics. It is used to model oscillatory systems, such as electromagnetic waves, and to solve differential equations. It also has applications in signal processing, quantum mechanics, and fluid dynamics.