Proof of e^(ix) = cos(x) + isin(x)
Date: 04/07/97 at 21:07:52 From: Walter Graf Subject: Proof of e^(ix) = cos(x) + isin(x) In the equation e^(iPi) - 1 = 0, the proof is to evaluate e^(ix) = cos(x) + isin(x) for x = Pi. I would like to see a rigorous proof of of the the above equation. Thank you, Walter Graf
Date: 04/08/97 at 05:16:59 From: Doctor Mitteldorf Subject: Re: Proof of e^(ix) = cos(x) + isin(x) Dear Walter, This is called the Euler equation, and it's not something you can prove rigorously. It's a definition, and I'd like to convince you that it's the only sensible definition, of how to compute imaginary exponentials. I can think of three approaches to verifying the Euler equation, but unfortunately one of them is all calculus, one uses calculus explicitly, and only the third is free of calculus. I'm just guessing from your age that you may not have studied calculus yet. You can verify that the Euler equation makes a sensible definition by expanding the two sides as Taylor series in x. You can also differentiate both sides and see that the answer is self-consistent. Thirdly, you can use the formula for cos(2x) and sin(2x) to show that the right side has the property you expect from an exponential, so that e^i(2x) = (e^ix)^2. So start with choice 3. You have the formulas cos(2x) = cos^2(x) - sin^2(x) and sin(2x) = 2 sin(x) cos(x) You'd also want to demand that e^i(2x) = (e^ix)^2. That means that your new definition of e^ix is behaving like an exponential. See if you can put these together to show that e^i(2x) = cos (2x) + i sin(2x). The Taylor expansion is something you can appreciate without calculus, although its roots are in calculus. It's a series expression for a function. You may have run across the following infinite series representations of cos and sin and e^x. In fact, this is the most straightforward way to compute the value of sin(x) or e^x for any given x. cos(x) = 1 - x^2/2! + x^4/4! - x^6/6! + ... sin(x) = x - x^3/3! + x^5/5! - x^7/7! + ... e^x = 1 + x + x^2/2! + x^3/3! + x^4/4! + ... The ! in these equations means factorial. In other words, 4! = 4*3*2*1. See if you can use these infinite series expressions to verify the Euler equation. After you complete these two projects, I'm hoping you'll find the Euler equation very plausible. If you still want more proof, write back again... -Doctor Mitteldorf, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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