I just solved on Khan Academy the following problem, which gives me on my calculator EL-W506X the wrong answer. Why is that?
$$\int_{1}^3{\ln(x+1)\cdot \sin(x) dx} = 1.57079$$ (says on Wolfram Alpha), but on my EL-W506X gives the value of $0.079306699$.
have you checked whether your calculator is on radian mode? Because wolfram will produce the result of $sin(3)$ for the units of radians, and your calculator might be using different units.