- #1

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Hi, I've already worked out the question until part (a) but I don't understand how they go from (a) to (b) right at the bottom of the image.

How did they change -s + i to s - i ?

[PLAIN]http://img64.imageshack.us/img64/9366/unledrze.png [Broken]

Thank you very much for your help.

How did they change -s + i to s - i ?

[PLAIN]http://img64.imageshack.us/img64/9366/unledrze.png [Broken]

Thank you very much for your help.

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