Solution 4.5a
From Mechanics
Using \displaystyle {{F}^{2}}={{H}^{2}}+{{V}^{2}}
and
\displaystyle \tan \alpha =\frac{V}{H}
Here
\displaystyle V=A and \displaystyle H=B.
Substituting the given values
\displaystyle \begin{align}
& {{F}^{2}}={{4}^{2}}+{{3}^{2}}=25 \\
& \text{giving} \\
& F=5\ \text{N} \\
\end{align}
Also
\displaystyle \tan \alpha =\frac{3}{4}=0\textrm{.}75
giving
\displaystyle \alpha =36\textrm{.}9{}^\circ