$f(x)=x^{1/2},f^\prime(x)=\frac{1}{2}x^{-1/2},f^{\prime\prime}(x)=-\frac{1}{4}x^{-3/2},f^{\prime\prime\prime}(x)=\frac{3}{8}x^{-5/2}$

Simplify.

Substitute x² + 1 for x into your equation from Step 3 to obtain the sixth-degree power series.