Sinusoidally Varying Signal

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I have seen that, to represent a signal of certain magnitude which is varying in time sinusoidally, we multiply the magnitude of that signal with e^(jwt). Why can't we just multiply the amplitude with a sine or cosine of the same frequency? What is the difference in these two representations?

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The form e^(jwt) is called time harmonic vector (field). It's convenient form, as derivates are replaced by multiplication by jw. In fact, you can go from exponential presentation to time dependent form by taking real part of exponential form*e^(jwt), which often results in cosine (or sine, depending on phase shift.)