JEE Advanced Physics: Continuity and Differentiability questions with solutions

Q1. If y = sin(x) + cos(x), find d²y/dx².

1. sin(x) + cos(x)
2. -sin(x) + cos(x)
3. -(sin(x) + cos(x))
4. None of these

Answer: -(sin(x) + cos(x))

Differentiating sin(x)+cos(x) once gives cos(x)-sin(x), and differentiating again gives -sin(x)-cos(x) = -(sin(x)+cos(x)).