$y' = -2x \cos x +(x^2-2)\sin x +2\sin x +2x\cos x=x^2\sin x $
b) $y'=4 \bigg ( \dfrac{2x^2+1}{x^2 -3}\bigg )^3 .\bigg ( \dfrac{2x^2+1}{x^2 -3}\bigg )'=-56x. \bigg ( \dfrac{2x^2+1}{x^2 -3}\bigg )^3.\dfrac{1}{(x^2-3)^2}$
c) $y'=\dfrac{(1+\cos^2 \dfrac{x}{2})'}{2 \sqrt{1+\cos^2 \dfrac{x}{2}}}=-\dfrac{\sin x }{4 \sqrt{1+\cos^2 \dfrac{x}{2}}}=-\dfrac{\sin x}{2\sqrt 2 \sqrt{3+\cos x}}$