## Elementary Geometry for College Students (6th Edition)

x$\approx$ 6.5
using theorem 11.4.3 Law of cosine $x^{2}$ = $(10)^{2}$ + $(9)^{2}$ - 2*10*9*cos 40° as we know cos 40° = 0.7660 $x^{2}$ = 100+81 - 180 * 0.7660 $x^{2}$ = 181 -180 * 0.7660 $x^{2}$ = 43.12 x = $\sqrt 43.12$ x$\approx$ 6.5