### Home > PC3 > Chapter 7 > Lesson 7.1.5 > Problem7-66

7-66.

Josh knows that the Law of Cosines is $c^2=a^2+b^2-\underline{\quad}ab\cos(θ)$, but he cannot remember whether $1$ or $2$ goes in the blank. Show him how he can use an equilateral triangle to decide.

Let $a=b=c=1$ (or any number).