Prove Theorem \(1.3\) (Schwarz): \(|u \cdot v| \leq\|u\|\|v\|\).
For any real number \(t\), and using Theorem \(1.2\), we have
$$
0 \leq(t u+v) \cdot(t u+v)=t^{2}(u \cdot u)+2 t(u \cdot v)+(v \cdot
v)=\|u\|^{2} t^{2}+2(u \cdot v) t+\|v\|^{2}
$$
Let \(a=\|u\|^{2}, b=2(u \cdot v), c=\|v\|^{2}\). Then, for every value of \(t, a
t^{2}+b t+c \geq 0\). This means that the quadratic polynomial cannot have two
real roots. This implies that the discriminant \(D=b^{2}-4 a c \leq 0\) or,
equivalently, \(b^{2} \leq 4 a c\). Thus,
$$
4(\boldsymbol{u} \cdot v)^{2} \leq 4\|u\|^{2}\|v\|^{2}
$$
Dividing by 4 gives us our result.