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- Thread starter Zula110100100
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- #2

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It doesn't look ok. Can you perhaps write the whole thing/problem ?

- #3

HallsofIvy

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The best you can do is use the sum identities:

sin(x+ dx)= sin(x)cos(dx)+ cos(x)sin(dx)

cos(x+ dx)= cos(x)cos(dx)- sin(x)sin(dx)

You could then argue that for dx sufficiently small sin(dx) is approximately equal to dx and cos(dx) is approximately equal to 1:

sin(x+ dx) is approximately sin(x)+ cos(x)dx

cos(x+ dx) is approximately cos(x)- sin(x)dx.

Now, r sin(x+ dx)cos(x+ dx) will be approximately

[itex]sin(x)cos(x)+ cos^2(x)dx- sin^2(dx)- sin(x)cos(x)dx^2[/itex]

I suppose you could not argue that, for dx a "differential", you can drop [itex]dx^2[/itex] to get [itex]sin(x)cos(x)+ cos^2(x)dx- sin^2(x)dx[/itex]

But those still will not give you something "times dx".

As dextercioby said, please show the entire problem.

- #4

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Thats ugly as hell, but I think you get the idea yes?

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