If  $\overline{a}and\overline{b}$ are unit vectors and $\overline{c}$  is a vector such that $\overline{c}=\overline{a}\times \overline{c}+\overline{b}$  then

$$\begin{gathered} \overline{c}.\overline{a}=(\left(\overline{a}x\overline{c}\right)+\overline{b}).\overline{a}=\overline{b}.\overline{a} \overline{b}x\overline{c}=(\overline{b}.\overline{c})\overline{a}-(\overline{a}.\overline{b})\overline{c} \\ \therefore \left[\overline{a}\overline{b}\overline{c}\right]=\overline{b}.\overline{c}-(\overline{a}.\overline{b})(\overline{a}.\overline{c})=\overline{b}.\overline{c}-{(\overline{a}.\overline{b})}^2 \end{gathered}$$

Also,  $\overline{c}.\overline{b}=1-\left[\overline{a}\overline{b}\overline{c}\right]$

$\therefore 2\left[\overline{a}\overline{b}\overline{c}\right]=1-{(\overline{a}.\overline{b})}^2\le 1$

$\therefore \left[\overline{a}\overline{b}\overline{c}\right] \le \frac{1}{2}$