分子
x->0
[ 1-(1/2)x^2+o(x^2)].[ 1-(1/2)(2x)^2+o(x^2)]
= 1-(1/2)x^2 -(1/2)(2x)^2 +o(x^2)
=1 -(5/2)x^2 +o(x^2)
[ 1-(1/2)x^2+o(x^2)].[ 1-(1/2)(2x)^2+o(x^2)] .[ 1-(1/2)(3x)^2+o(x^2)]
=[1 -(5/2)x^2 +o(x^2)].[ 1-(1/2)(3x)^2+o(x^2)]
=1 -(5/2)x^2 -(1/2)(3x)^2+o(x^2)
=1- 7x^2 +o(x^2)
1- [ 1-(1/2)x^2+o(x^2)].[ 1-(1/2)(2x)^2+o(x^2)] .[ 1-(1/2)(3x)^2+o(x^2)]
=7x^2+o(x^2)
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