sin(270-theta)= -cos theta

sin(270-theta) = -cos theta is right.

 

Because in in unit circle 270-theta means u will in third qua. & in third qua. sin function is negative. 

what we have to do then

As u know at the Y-axis function will change. So here sin function will convert in to cos function. Sin function is -ve int 3^rd qua. so cos function will become -ve.

 

Like, I have sin(240) = sin(270-30) = -cos30

aadit how do u know what to do

Because i have alredy studied it in class 11.