#### Answer

(a) $A\cdot B = 11.5$
(b) $C\cdot D = -4.6$
(c) $E\cdot F = 0$

#### Work Step by Step

(a) $A\cdot B = \vert {A}\vert ~\vert {B}\vert~cos(\theta)$
$A\cdot B = (3)(5)~cos(40^{\circ})$
$A\cdot B = 11.5$
(b) $C\cdot D = \vert {C}\vert ~\vert {D}\vert~cos(\theta)$
$C\cdot D = (2)(3)~cos(140^{\circ})$
$C\cdot D = -4.6$
(c) $E\cdot F = \vert {E}\vert ~\vert {F}\vert~cos(\theta)$
$E\cdot F = (3)(4)~cos(90^{\circ})$
$E\cdot F = (3)(4)(0)$
$E\cdot F = 0$