Tolong caranya dong kak

Tolong caranya dong kak

$vec{a}. vec{b} = | vec{a}| | vec{b}| cos( alpha ) \ (1 times - 1) + (1 times 0) + 0 times 1) = sqrt{ {1}^{2} + {1}^{2} + {0}^{2} } times sqrt{ {( - 1)}^{2} + {0}^{2} + {1}^{2} } cos( alpha ) \ - 1 = sqrt{2} sqrt{2} cos( alpha ) \ - 1 = 2 cos( alpha ) \ cos( alpha ) = - frac{1}{2} \ alpha = 120 degree \ sin( alpha ) = sin(120 degree) = frac{1}{2} sqrt{3}$