x_{123} = \frac{\frac{L}{4} + 2 \frac{L}{4} + 3 \frac{L}{4}}{3} = 6 \frac{L}{12} = \frac{L}{2}

${x_{123} = \frac{\frac{L}{4}+2\frac{L}{4}+3\frac{L}{4}}{3} = 6\frac{L}{12} = \frac{L}{2}}$

Error