kaisan
verify: sin(x+y)sin(xy)=2cosxsiny



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malexander
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cos(x+y)=cosxacosysinxsiny
cos(xy)=cosxcosy+sinxsiny
addition produces 2cosxcosy

lgbasallote
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take the left hand side...do you know the formula for sin(x+y) and sin(xy)?

malexander
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Its an identity.

kaisan
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i think so but i'm not sure

lgbasallote
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sin(x+y) = sinxcosy + sinycosx
sin(xy) = sinxcosy  sinycosx
can you do it from here?

kaisan
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it looks familiar but i can't figure it out. is it one of the cosine identities?

lgbasallote
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no...you substitute those into your problem
sin(x+y)  sin (xy) = sinxcosy + cosxsiny  (sinxcosy  cosxsiny)

kaisan
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i'm still confused

lgbasallote
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with what?

kaisan
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how to do all of this in general

lgbasallote
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basically you memorize the identities...

lgbasallote
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i did not do anything fancy...all i did was substitute identities

kaisan
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oh ok

lgbasallote
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so regarding this problem...you're good?

kaisan
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not really...

lgbasallote
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sinxcosy + cosxsiny  (sinxcosy  cosxsiny)
sinxcosy = cosxsiny  sinxcosy + cosxsiny

lgbasallote
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that's a + not an =

kaisan
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i don't understand what you did

lgbasallote
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i distributed the minus sign

kaisan
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oh ok i see

lgbasallote
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so you're good now?

kaisan
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yes thanks ^^