We know
#(costheta+sintheta)^2+(costheta-sintheta)^2=2(cos^2theta+sin^2theta)#
#=>(sqrt2costheta)^2+(costheta-sintheta)^2=2xx1#
#=>(costheta-sintheta)^2=2-2cos^2theta#
#=>(costheta-sintheta)^2=2(1-cos^2theta)#
#=>(costheta-sintheta)^2=2sin^2theta#
#=>costheta-sintheta=sqrt2sintheta#