Prove: -) cos p +cos q =2cos(p+q/2)*cos(p-q/2) <<< As written, this is not an identity. For identityt, it should be written as:
cos p +cos q =2cos[(p+q)/2] * cos[(p-q)/2]
Notice
if
P = A + B
and
Q = A - B
then
A = (P+Q)/2
and
B = (P-Q)/2
expand
cos(P) = cos(A+B) = cos(A) * cos(B) - sin(A) * sin(B)
Now continue....
As a general convention, for names of angles - capital letters are used (such as A, B, etc.) - and -
for names of sides of polygons - small letters are used (such as 'a', 'b', etc.)
:knowing that a+b=p and a-b=q
Thanks in advance
