<img src="https://tex.z-dn.net/?f=T%20%3D%20%202%20%5Cpi%20%5Csqrt%7B%20%5Cfrac%7Bl%7D%7Bg%7D%7D%20" />
<img src="https://tex.z-dn.net/?f=%28%5Csqrt%7B%5Cfrac%7BT%7D%7B2%20%5Cpi%7D%7D%20%29%5E%7B2%7D%20%3D%20%20%5Cfrac%7Bl%7D%7Bg%7D%20" />
l = g<img src="https://tex.z-dn.net/?f=%28%20%5Csqrt%7B%20%5Cfrac%7BT%7D%7B2%20%5Cpi%7D%7D%20%29%5E%7B2%7D%20%20" />
<img src="https://tex.z-dn.net/?f=%20l_%7BB%7D%20%3D%209.795%20%28%5Csqrt%7B%20%5Cfrac%7B4%7D%7B2%20%5Cpi%7D%7D%29%5E%7B2%7D%20" />
<img src="https://tex.z-dn.net/?f=%20l_%7BB%7D%20%3D%203.9698%20m%20" />
<img src="https://tex.z-dn.net/?f=%20l_%7BQ%7D%20%3D%209.780%20%28%5Csqrt%7B%20%5Cfrac%7B4%7D%7B2%20%5Cpi%7D%7D%29%5E%7B2%7D%20" />
<img src="https://tex.z-dn.net/?f=%20l_%7BQ%7D%20%3D%203.9637%20m%20" />
∴Δl = <img src="https://tex.z-dn.net/?f=%20l_%7BB%7D%20-%20%20l_%7BQ%7D%20%20" /> Δl = 6.
1x10 ^ - 3 m Δl = 6.
1 mm.