Fk(x)=(1+1−4x)k−(1−1−4x)k(1+1−4x)k+1−(1−1−4x)k+1F_k(x)=\frac{(1+\sqrt{1-4x})^k-(1-\sqrt{1-4x})^k}{(1+\sqrt{1-4x})^{k+1}-(1-\sqrt{1-4x})^{k+1}}Fk(x)=(1+1−4x)k+1−(1−1−4x)k+1(1+1−4x)k−(1−1−4x)k
这个的分母该怎么展开呢？
