a=2i-j+2k
b=-i+0j +2k
axb =(2i-j+2k)x(-i+0j +2k)
= 2i x(-i+0j +2k) - j x(-i+0j +2k) + 2k x(-i+0j +2k)
=(-4j) - (k+2i ) +(-2j)
=-2i-6j -k
|axb| =√(2^2+6^2+1^2) =√41
a,b为邻边的平行四边形的面积
=|axb|
=√41