To find the error, we repeat the steps correctly:
i)
![b=(1)/(2)\cdot\sqrt[]{a-1}](https://img.qammunity.org/2023/formulas/mathematics/college/lx5sus0tuw2zs7dbrl61f5csp6c8walqfo.png)
ii) We pass the 2 dividing on the right, multiplicating at the left:
![2b=\cdot\sqrt[]{a-1}](https://img.qammunity.org/2023/formulas/mathematics/college/k5yy2rnp74mgqu2sdwkm8hrtm2sy5sk3fr.png)
iii) We take the square on both sides:
![\begin{gathered} (2b)^2=(\sqrt[]{a-1})^2 \\ 2^2b^2=a-1 \\ 4b^2=a-1 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/9sp6m6r5muowiwzqy8onalcamheono4x8c.png)
iv) Finally we pass the -1 on the right as +1 to the left:

Answer
Comparing the steps, we see that the error was in step III, the square of 2b is 4b², not 2b².