mathslover
sin ( 90 + theta ) = cos theta
prove^



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anonymous
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i guess it depends on what you can use
if you can use the addition angle formula, that would work
if not maybe with a triangle you can see it

mathslover
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i want to do with the triangle one but m wondering how will you show (90+theta) in a right angled triangle?

anonymous
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dw:1350098235823:dw

mathslover
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yes but that is 90  theta? do 90theta has any relation with 90 + theta ?

anonymous
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\[\cos(x)=\frac{a}{c}=\sin(90x)=\sin(x90)\]
\[=\sin(x90+180)=\sin(x+90)\]

mathslover
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how : sin(x90) = sin(x90+180) ?

anonymous
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third equal sign because sine is odd, fourth because \(\sin(x)=\sin(x+180)\)

mathslover
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Gotcha now, thanks a lot!

anonymous
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your on the other side of the unit circle dw:1350098472701:dw

anonymous
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yw

mathslover
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hey, how you did this step? :
\[\large{\sin(x90) = sin(x90+180) }\]
?

mathslover
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@satellite73 did you subtract sin(180) both sides?

anonymous
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i added 180 inside because \(\sin(x)=\sin(x+180)\)

mathslover
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oh k , got it!