※ 引述《oxs77 (安)》之銘言：
: 已知 cosx + cosy =1/2
: sinx + siny =1/3，
: 則cos (x + y) = ?
Let a=cosx+Isinx, b=cosy+Isiny
a'=1/a, b'=1/b are conjugation of a and b respectively.
then a+b=1/2+I/3
and a'+b'=1/2-I/3, ie. (a+b)/ab = 1/2-I/3
Thus, ab=(1/2+I/3)/(1/2-I/3)=5/13 + I*12/13
Hence, cos(x+y)=5/13.
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