Cosinus sudut antara vektor a = 2i + 2j + k dan b = 6i + 2j – 3k adalah

cos alfa = (a.b)/(|a|.|b|)
cos alfa = ((2,2,1) . (6,2,-3))/(akar2²+2²+1²).(akar6²+2²+(-3)²)
cos alfa = (12+4-3)/(akar4+4+1).(akar36+4+9)
cos alfa = 13/(3*7)
cos alfa = 13/21