Posted by Yul on Monday, November 14, 2011 at 12:23am.
cosθ =5/13 with π/2 < θ <0 find each of the following
sin( θ  5π/4)
cos( θ  11π/6)
tan( θ + 5π/4)
can some1 explain to me how to do these? im completely lost and this is the only thing about my assignment that i cant do can someone please help me?

trig  Steve, Tuesday, November 15, 2011 at 12:34am
Ah, finally a legible posting! :)
This is an exercise in the addition/subtraction formulas.
First off, it is clear that θ is in the 4th quadrant, so
sinθ = 12/13
tanθ = 12/5
sin(ab) = sina cosb  cosa sinb
so,
sin(θ  5π/4) = sinθ cos 5π/4  cosθ sin 5π/4
= (12/13)(1/√2)  (5/13)(1/√2)
= 17/(13√2)
Do the others the same way, using the appropriate addition formula.
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