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Unformatted text preview: MA 36600 LECTURE NOTES: WEDNESDAY, FEBRUARY 25 Complex Roots Eulers Formula. We showed in the previous lecture that e it = cos t + i sin t. This is known as Eulers Formula . In general, if we write r = + i in terms of real numbers and , we have the expression e rt = e t e it = e t (cos t + i sin t ) = e t cos t + ie t sin t. In fact, because we have the Taylor Series expansion e rt = X k =0 r k k ! t k = d dt e rt = r e rt for any complex number r . Hence we can always make sense of the function y ( t ) = e rt as a solution to a homogeneous linear differential equation with constant coefficients. Example. We explain how to find the general solution to the differential equation y 00 + 9 y = 0 . We guess a solution in the form y ( t ) = e rt so that we have the characteristic equation r 2 + 9 = 0 = r = 3 i. Hence the general solution is the function y ( t ) = a 1 e 3 i + a 2 e 3 i = a 1 (cos3 t + i sin3 t ) + a 2 (cos3 t i sin3 t ) = c 1 cos3 t + c 2 sin3 t in terms of the constants c 1 = a 1 + a 2 and c 2 = ia 1 ia 2 . Review. Consider the constant coefficient second order differential equation ay 00 + by + cy = 0 . Recall that it has the associated characteristic equation ar 2 + br + c = 0 . We assume that b 2 4 ac < 0 so that we have complex roots. We may write these complex roots as r 1 = + i r 2 =  i in terms of = b 2 a , = p  b 2 4 ac  2 a . We saw in the previous lecture that we have the expression e r 1 t = e t e it = e t (cos t + i sin t ) = e t cos t + ie t sin t ; e r 2 t = e t e it = e t (cos t i sin t ) = e t cos t ie t sin t ....
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 Spring '09
 MA
 Real Numbers

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