Without using the derivative, show that the function `f(x)=|x|` is (a) strictly increasing in `(0,\ oo)` (b) strictly decreasing in `(-oo,\ 0)`

Explanation

(a) f(x)=∣x∣

let `x_1​,x_2`​ be two function

`x_1​=x`

`x_2​=x+a
`

where a>0

`f(x_2​)=∣x_1​+a∣=X_1​+a`

`f(x_1​)=X_1​`

...