SOLUTION: If sin theta - cos theta =0 then the value of (sin^4theta + cos^4 theta)is ?

 = 0  ====>   =   ====>  (divide by  both sides )  ====> 


 = 1  ====>   =   or   = .


So, EITHER  =  =   OR

            =  = .


In any case,   =  =  = .