Question #b35f7

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Answer 1 · 2017-07-17T06:50:02

The angle is $=2/3pi$

$veca$ and $(veca+vecb)$ are perpendicular

Therefore, the dot product is $=0$

$veca.(veca+vecb)=veca.veca+veca.vecb=0$

$veca.veca=||veca||^2$

But,

$||veca||=1/2||vecb||$

The angle between $veca$ and $vecb$ is given by the dot product

$veca.vecb=||veca||*||vecb||costheta$

$costheta=(veca.vecb)/(||veca||*||vecb||)$

$=(-||veca||^2)/(2||veca||^2)$

Therefore,

$theta=2/3pi$