Question

plaease solve this Q

Answer 1

$as \: we \: know \\ \frac{du}{dx} = - f \\ from \: 0 \: to \: x1 \\ \frac{du}{dx} = - f(negative)(slope \: increaing) \\ \frac{du}{dx} = 0 \: line \: is \: horizontal \: so \: slope \: is \: zero \: hence \: f = 0 \\ in \: last \: case \\ \frac{du}{dx} = - ve \\ but \: f = \frac{ - du}{dx} \\ hence \: it \: will become \: + ve$