Which of the following functions isare solutions to the

Question

Which of the following functions is/are solutions to the differential equation (y')^2 - 1 + y^2? Note: There may be more than one solution.

A. y(x) = (cos(x) + sin(x))/ sqrt(2)

B. y(x) = cos(x)

C. y(x) = (cos(x) - sin(x)) / sqrt(2)

D. y(x) = 1/2 (cos(x))