## Elementary Technical Mathematics

The measure of angle A is $44{}^\circ$
Here, $a=18.2cm,b=20.5cm$ and $c=26.1cm$ Use the law of cosines to find the measure of angle A as, ${{a}^{2}}={{b}^{2}}+{{c}^{2}}-2bc\text{ }\cos A$ …… (1) Substitute the values of a, b and c in equation (1), \begin{align} & {{\left( 18.2 \right)}^{2}}={{\left( 20.5 \right)}^{2}}+{{\left( 26.1 \right)}^{2}}-2\left( 20.5 \right)\left( 26.1 \right)\text{ }\cos A \\ & 331.24=416.16+681.21-\left( 1070.1 \right)\text{ }\cos A \\ & 331.24=1097.37-\left( 1070.1 \right)\text{ }\cos A \end{align} Subtract 1097.37 from both sides: \begin{align} & 331.24-1097.37=1097.37-\left( 1070.1 \right)\text{ }\cos A-1097.37 \\ & -766.13=-\left( 1070.1 \right)\text{ }\cos A \end{align} Divide both sides by $-1070.1$ to isolate $\cos A$, \begin{align} & \frac{-766.13}{-1070.1}=\frac{-1070.1}{-1070.1}\text{ }\cos A \\ & 0.7159=\cos A \end{align} Therefore, the value of A is, \begin{align} & A={{\cos }^{-1}}\left( 0.7159 \right) \\ & =44{}^\circ \end{align} Therefore, the measure of angle A is $44{}^\circ$.