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March 29, 2015

Posted by **Jerry** on Tuesday, November 13, 2012 at 9:47am.

I am working with one side but I am not sure if I can subtract at all here.

- Prove or disprove? -
**Reiny**, Tuesday, November 13, 2012 at 10:08amTo see if it is an identity, pick any value of x

e.g. x = 78°

then x+π/6 ---> 78+30 = 108°

and x-π/3 ---> 78 - 60 = 18°

is cos108 + sin18 = 0 ???

YES

ok, then , let's prove it

LS = cosxcosπ/6 - sinxsinπ/6 + sinxcosπ/3 - cosxsinπ/3

= √3/2 cosx -1/2 sinx + 1/2 sinx - √3/2 cosx

= 0

= RS

- Prove or disprove? -
**Steve**, Tuesday, November 13, 2012 at 10:23amor, since we know that cos(pi/2 - x) = sin(x)

cos(pi/2 - (x+pi/6)) = sin(pi/3 - x) = -sin(x-pi/3)

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