|[3a-b]*[a-2b]| = | [3a,a] - [3a,2b] - [b, a] +[b,2b]| =
= |3*<u>0</u>- (6)*<u>[</u>a,b] - [b,a] +2*<u>0</u>| = | - (6)*<u>[</u>a,b] + [a,b] | =
= | - (5)*<u>[</u>a,b] | =|-5*|a| *|b|*sin(π/2) | = | -5*3*4*1| = |-60| = 60
<u>( </u>в решении <u> 0</u> обозначен нулевой вектор) ,