Problem 1.236

A force $\mathbf{F}_ {1}=A \mathbf{j}$ is applied to a point whose radius vector $r_{1}=a i$, while a force $F_{2}=B i$ is applied to the point whose radius vector $\mathbf{r}_ {2}=b \mathbf{j} .$ Both radius vectors are determined relative to the origin of coordinates $O, \mathbf{i}$ and $\mathbf{j}$ are the unit vectors of the $x$ and $y$ axes, $a, b, A, B$ are constants. Find the arm $l$ of the resultant force relative to the point $O$.