Question

Two sides of a triangle are $3$ and $1$ units and their included angle is $60$ degrees. What is the third side?

• A. $5$
• B. $\sqrt{5}$
• C. $7$
• D. $\sqrt{7}$

Answer 1

The correct option is D $\sqrt{7}$

From cosine rule;
$a^2=b^2+c^2-2bccosA$
In the given problem $A={60}^o,b=3,c=1$

$\therefore a^2=3^2+1^2-2\left(3\right)\left(1\right)cos{60}^o$
$\therefore a=\sqrt{7}$