\color{rgb(0,0,0)}X \text{ suit la loi normale }N\left( \mu;\sigma^2 \right) = N(0 ;1^2)
$$\color{rgb(0,0,0)}a=$$
$$\color{rgb(0,0,0)}= 1$$
$$\color{rgb(0,0,255)}a
$$
$$\color{rgb(0,0,0)}b=$$
$$\color{rgb(0,0,0)}= 3$$
$$\color{rgb(0,0,255)}b$$
$$\color{rgb(127,0,127)}P\left( {1 \le X \le 3 } \right) \approx 0.1573 $$
$$\color{rgb(0,0,0)}\mu = $$
$$\color{rgb(0,0,0)}= 0$$
$$\color{rgb(0,0,0)}\sigma=$$
$$\color{rgb(0,0,0)}= 1$$
$$\color{rgb(0,0,255)}\Rightarrow P(X \le a)$$
$$\color{rgb(0,0,255)}\Rightarrow P( X \ge a)$$
$$\color{rgb(0,0,255)}\Rightarrow P(a \le X \le b)$$