The exact value of cos(pi/4) is 1/square root 2
but when working out cos (-7pi/4) why is the answer square root 2/2 ?

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cos is an even function \[\huge\rm Cos(-x)=\cos(x)\]
what is cos at 7pi/4 radi ??

I have no idea, can you explain it to me further. I got pi/4

so how did you get pi/4 ?? :=)

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