$8\mathrm{.}3$ A bracket, as shown in Fig. $8\mathrm{.}74,$ is welded to

a plate. The welds have the same size, and the

permissible force per mm of the weld$-$length

is $1$ kN $.$ Calculate the lengths $l_1$ and $l_2.$

$[40$ and $80$ mm $]$

Fig. $8\mathrm{.}74$