Question

若丨→a丨=2,丨→b丨=5,则＜→a,→b＞=2π/3,则（2→a- →b）·→a=

Answer 1

解

a*b=|a||b|cos

=2×5×cos(2π/3)

=10×(-1/2)

=-5

∴(2a-b)a

=2a²-a*b

=2×4-(-5)

=8+5

=13