>>370
|A×B|^2 = |A|^2 |B|^2 (sinθ)^2 = |A|^2 |B|^2 {1-(cosθ)^2}
= |A|^2 |B|^2 - (|A||B|cosθ)^2
= |A|^2 |B|^2 - |A・B|^2