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