a. $x^{2} = 6,5^{2} + 5^{2} - 2. 6,5 . 5. cos72^{\circ}$ $\approx$ 47,16
$\Rightarrow$ x $\approx$ 6,87
b. $x^{2} = (\frac{1}{5})^{2} + (\frac{1}{3})^{2} - 2. \frac{1}{5}. \frac{1}{3}. cos123^{\circ}$ $\approx$ 0,22
$\Rightarrow$ x $\approx$ 0,47
a. $x^{2} = 6,5^{2} + 5^{2} - 2. 6,5 . 5. cos72^{\circ}$ $\approx$ 47,16
$\Rightarrow$ x $\approx$ 6,87
b. $x^{2} = (\frac{1}{5})^{2} + (\frac{1}{3})^{2} - 2. \frac{1}{5}. \frac{1}{3}. cos123^{\circ}$ $\approx$ 0,22
$\Rightarrow$ x $\approx$ 0,47