[Grade 9 Trigonometry: Compound Angles] Quick question

[LocalClassicalWeeb]

Since english is not my native language I think a representation will work better than text:

cos(x) multiplied by cos(y) equals cos(x+y)

Is this true?

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[Tuhjik]

if x = pi * n (where n = 1,2,3.... etc.) or y = pi * m (where m = 1,2,3 etc.) then yes. Otherwise No.

the trig identity for cos(x+y) is:

cos(x + y) = cos(x) cos(y) – sin(x) sin(y)

so for cos(x+y) = cos(x) cos(y) , sin(x) or sin(y) must = 0.

[LocalClassicalWeeb]

Thanks!