[MOVED] Trigonometric Identities: cos(-A) = cos (A), etc

[micheleab]

I need help with working through trigonometric identities...i have the formulas but i just do not know how to apply them correctly..

for example: Prove the identity:
a) cos(-A) = cos (A)
b) Sin (-A) = Sin (A)

for the cos one i tried using the addition identity but it did not solve correctly
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Edited by stapel -- Reason for edit: moved to appropriate category.

 

[skeeter]

cos(-A) =
cos(0 - A) =
cos(0)cos(A) + sin(0)sin(A) =
(1)cos(A) + (0)sin(A) =
cos(A) + 0 =
cos(A)

the second one should be sin(-A) = -sin(A)

sin(-A) =
sin(0 - A) =
sin(0)cos(A) - cos(0)sin(A) =
(0)cos(A) - (1)sin(A) =
0 - sin(A) =
-sin(A)

in future, post trig problems in the trig section.