How do you prove $cos(pi/2 + theta) = -sin(theta)$ ?

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Answer 1 · 2016-02-29T05:53:54

$cos(pi/2+theta)=-sintheta$ is proved by the formula $cos (a+b)=cosacosb-sinasinb$ .

$cos (a+b)=cosacosb-sinasinb$

let $a=pi/2 & b= theta$

$=>cos(pi/2+theta)=cos(pi/2)cos(theta)-sin(pi/2)sin(theta)$

$=>cos(pi/2+theta)=(0)costheta-(1)sintheta$

$=>cos(pi/2+theta)=0-sintheta$

$=>cos(pi/2+theta)=-sintheta$

How do you prove cos(pi/2 + theta) = -sin(theta)cos(pi/2 + theta) = -sin(theta)?