x+y=√2sin(a+n/4)
（x+y）^2=2sin(a+n/4)
x^2+2xy+y^2=2sin(a+n/4)(1)
x-y=√2sin(a-n/4)
(x-y)^2=2sin(a-n/4)
x^2-2xy+y^2=2sin(a-n/4)(2)
(1)+(2),并化简得,x^2+y^2=2sina*cos（n/4）
题的条件全么?