(x+y)^2=1+3xy
(x-y)^2=1-xy
u=(x+y)(x-y)
|u|=√(x+y)^2√(x-y)^2
=√(1+3xy)√(1-xy)
=√[-3(t-1/3)^2+2/3]
≤√6/3
故-√6/3≤u≤√6/3