Actually, 0 degrees is not a counterexample for the expression sin(y)*tan(y) = cos(y).
To see why, let's substitute y = 0 degrees into the expression:
sin(y)*tan(y) = cos(y)
sin(0)*tan(0) = cos(0)
0*tan(0) = 1
0 = 1
As we can see, the equation does not hold for y = 0 degrees. However, this does not make 0 degrees a counterexample, because 0 degrees is not in the domain of the tangent function.
The tangent function is undefined at odd multiples of 90 degrees (e.g. 90, 270, etc.), because at those angles the denominator of the tangent function becomes zero. Therefore, we cannot substitute y = 0 degrees into the expression sin(y)*tan(y) = cos(y), because it would result in division by zero.
In summary, 0 degrees is not a counterexample for the expression sin(y)*tan(y) = cos(y), because it is not in the domain of the tangent function.