解:(1)
x1 = y1
x2 = y2
x3 = y3+y4
x4 = y3-y4
f = y1^2+y2^2+y3^2-y4^2
(2)
x1 = y1+y2
x2 = y1-y2
x3 = y3+y4
x4 = y3-y4
x5 = y5
f = y1^2+y2^2+y3^2-y4^2+y3y5-y4y5
= y1^2+y2^2+(y3+(1/2)y5)^2 -(y4+(1/2)y5)^2
= z1^2+z2^2+z3^2-z4^2
其中
z1=y1
z2=y2
z3=y3+(1/2)y5
z4=y3-(1/2)y5
z5=y5
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