A reflection refers to the change that is made in the matrix of the function, we can also call it transformation.
To make a reflection we take the matrix of the function either f (x) = A sin (bx + c) + d, or cos in its same order.
And what we do is alter one of the values an example of it if A is 2Π we can change it to -2Π which would mean a reflection in one of the axes according to the positive or negative value that we give to Y, which is the same as saying f (x).
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