Answer
$-20$
Work Step by Step
We know that the angle between two vectors is:
$\theta =\cos^{-1} [ \dfrac{u \cdot v}{||u|| \space ||v|| }]$
Therefore, $\cos (\dfrac{2 \pi}{3}) = \dfrac{u \cdot v}{(4)(10) }$
or, $u \cdot v= 40 \times (-\dfrac{1}{2})$
or, $u \cdot v=-20$
