Laplace transform problem at the moment, and I'm a little stuck. ccL{sin(2x)cos(5x)}. I don't recall any trig identity that would apply here.

vidamuhae

vidamuhae

Answered question

2022-11-19

Laplace transform problem at the moment, and I'm a little stuck.
L { sin ( 2 x ) cos ( 5 x ) }
I don't recall any trig identity that would apply here. I know that
sin ( 2 x ) = 2 sin ( x ) cos ( x )
But I'm not sure if that applies in this situation.

Answer & Explanation

boursecasa2je

boursecasa2je

Beginner2022-11-20Added 15 answers

By the addition theorem for the sine, we can write
sin ( 7 x ) = sin ( 5 x + 2 x ) = sin ( 5 x ) cos ( 2 x ) + sin ( 2 x ) cos ( 5 x ) sin ( 3 x ) = sin ( 5 x 2 x ) = sin ( 5 x ) cos ( 2 x ) sin ( 2 x ) cos ( 5 x ) ,
and hence
sin ( 2 x ) cos ( 5 x ) = 1 2 ( sin ( 7 x ) sin ( 3 x ) ) .
Therefore
L { sin ( 2 x ) cos ( 5 x ) } = 1 2 ( L { sin ( 7 x ) } L { sin ( 3 x ) } ) .

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