Students forget to apply the chain rule when dealing with composite functions. For example, when differentiating sin(2x), many write cos(2x) instead of 2cos(2x). Always check if there's an 'inner function' and multiply by its derivative.
Another frequent error is sign mistakes in trigonometric derivatives. Remember that d/dx(cos x) = -sin x (negative sign), not +sin x.
For NDA success, practice mixed problems daily.
Focus on recognizing patterns quickly - whether to use power rule, product rule, or chain rule. Speed and accuracy in basic derivatives will save precious exam time for complex application problems.