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Maybe it is a very stupid question but I am having trouble with summing complex numbers in Mathematica. I have

q = l1 E^(2 π I t1) + l2  E^(2 π I t2)

where l1, l2 t1, and t2 are known real numbers. I obtain an answer which is a set of four complex numbers.

I think it sums all the components one by one (Minkowski sum?), and I want just to sum two complex numbers to obtain a third one. What function should I use?

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    $\begingroup$ You might want to look at ComplexExpand[]. $\endgroup$ – J. M. will be back soon Jul 11 '15 at 12:29
  • $\begingroup$ What do you mean by "2[Pi]"? $\endgroup$ – Peltio Jul 11 '15 at 12:30
  • $\begingroup$ @Peltio, Markdown strips backslashes, unfortunately. I've fixed it. $\endgroup$ – J. M. will be back soon Jul 11 '15 at 12:37
  • $\begingroup$ q // ExpToTrig // Simplify is equivalent to q//ComplexExpand in this case. $\endgroup$ – Bob Hanlon Jul 11 '15 at 13:16
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Since l1, l2, t1, t2 are known, you just need to plug them in:

q=l1 E^(2 \[Pi] I t1) + l2 E^(2 \[Pi] I t2) /. {l1->0.2, t1->0.1, l2->5, t2->-1}

gives 5.1618 + 0.117557 I

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   q1 = l1 E^(2 \[Pi] I t1) + l2 E^(2 \[Pi] I t2)

        q2 = q1 // ExpToTrig

        (Drop[q2, 2] // Factor) + Drop[q2, -2]

        (*  E^(2 I \[Pi] t1) l1 + E^(2 I \[Pi] t2) l2

        l1 Cos[2 \[Pi] t1] + l2 Cos[2 \[Pi] t2] + I l1 Sin[2 \[Pi] t1] + 
         I l2 Sin[2 \[Pi] t2]

        l1 Cos[2 \[Pi] t1] + l2 Cos[2 \[Pi] t2] + 
         I (l1 Sin[2 \[Pi] t1] + l2 Sin[2 \[Pi] t2])  *)

Have fun!

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