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a)
D\ \frac{2}{\cos x}=\frac{-\sin x-\cos x}{\cos^2x}
b)
D\ \frac{2x}{\cos x}=\frac{2\cos x+2x\sin x}{\cos^2x}=\frac{2\left(\cos x+x\sin x\right)}{\cos^2x} 
c)
D\frac{\cos x}{2}=\frac{-2\sin x}{4}=\frac{-\sin x}{2}