Question #fca5d

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Answer 1 · 2017-04-17T08:37:01

$"theta=pi/2$

$"you can use the cosine law."$
$r^2=a^2+b^2+2ab*cos theta$

$15^2=9^2+12^2+2*15*9*cos theta$

$225=81+144+270*cos theta$

$225=225+270*cos theta$

$225-225=270*cos theta$

$0=270*cos theta$

$cos theta =0$

$arc cos( 0)=pi/2$

$theta=pi/2$