令t=sinx+cosx
则t^2=1+2sinxcosx,得sinxcosx=(t^2-1)/2
y=sinxcosx+√2(sinx+cosx)+2
=(t^2-1)/2+√2t+2
=(t^2+2√2t+3)/2
=[(t+√2)^2+1]/2
因为t=sinx+cosx=√2sin(x+π/4),故|t|