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[wgy182]

Show that cos(x+y)cos(x-y)=cos^2(x)-sin^2(y)


Thanks in advance

 

[Carrotsticks]

Just expand cos(x+y)=cos(x)cos(y)-sin(x)sin(y) and similarly for cos(x-y), then expand and simplify (since you don't learn Products to Sums formula)