>>56
(1+x)^(1/2)=|a+1|/√(1+a^2)
(1−x)^(1/2)=|a−1|/√(1+a^2)

|a+1|=a+1(a≧−1),−a−1(a≦−1)
|a−1|=a−1(a≧1),−a+1(a≦1)

-1<a<1では
|a+1|=a+1,|a−1|=−a+1

(1+x)^(1/2) +(1−x)^(1/2)
=2/(1+a^2)