If $Cos(theta)= 2/3$ then what is $cos(theta + pi)$ ?

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Answer 1 · 2016-10-04T15:53:22

$cos(theta+pi)=-2/3$

Use the formula cos(A+B)=cosAcosB-sinAsinB.So
$cos(theta+pi)=cos theta cos pi -sin theta sin pi$

$=2/3 (-1)-sin theta (0)$

Note that we can find the sin of theta from the given ratio but since sine of pi is 0 we don't have to bother finding it.

$=-2/3 -0$

$=-2/3$

If Cos(theta)= 2/3Cos(theta)= 2/3 then what is cos(theta + pi)cos(theta + pi)?