$T=2 \pi \sqrt{\frac{L}{g}}$

$\Rightarrow T^2=4 \pi^2 \frac{L}{g}$

$\frac{\Delta g}{g} \times 100 \%=\frac{\Delta L}{L} \times 100 \%+\frac{2 \Delta T}{T} \times 100 \%$

$\frac{\Delta g}{g} \times 100 \%=(\alpha+2 \beta) \times 100$