a. Ta có: \(y' = {1 \over {2\left( {a + b} \right)\sqrt x }} \Rightarrow dy = {1 \over {2\left( {a + b} \right)\sqrt x }}dx\)
b. \(y' = \sin x + x\cos x\)
\(\Rightarrow dy = y'dx = \left( {\sin x + x\cos x} \right)dx\)
c. \(dy = y'dx = \left( {2x + \sin 2x} \right)dx\)
d. \(dy = y'dx = 3{\tan ^2}x\left( {1 + {{\tan }^2}x} \right)dx\)
