Question

For triangle ABC, a = 79, b = 68, c = 61, then M∠A= ___ °Round answer to one decimal place.

Answer 1

Given a triangle ABC below

To find the value of m∠A, we use the cosine formula which is

$\cos A=\frac{b^2+c^2-a^2}{2bc}$

Where

$\begin{gathered} a=79 \\ b=68 \\ c=61 \end{gathered}$

Substitute the values into the cosine formula above

$\begin{gathered} \cos A=\frac{b^2+c^2-a^2}{2bc} \\ \cos A=\frac{68^2+61^2-79^2}{2(68)(61)} \\ \cos A=\frac{2104}{8296} \\ \cos A=0.2536 \\ A=\cos ^{-1}(0.2536) \\ A=75.3\degree\text{ (one decimal place)} \end{gathered}$

Hence, m∠A is 75.3° (one decimal place)