The period of a pendulum is related to its frequency by the following formula:
![T=(1)/(f)](https://img.qammunity.org/2023/formulas/physics/college/6lpq7lktxhyd0kjajs047cwrdxsfjjia47.png)
If the frequency is halved we have:
![T_2=(1)/((f)/(2))](https://img.qammunity.org/2023/formulas/physics/college/7fddy725m1xu8z894ljalwlpc8e46nro2i.png)
Simplifying we get:
![T_2=2((1)/(f))](https://img.qammunity.org/2023/formulas/physics/college/2296wjonkpblicxz44hf49gu7k83ycjund.png)
The term inside the parenthesis is the original period, therefore:
![T_2=2T](https://img.qammunity.org/2023/formulas/physics/college/1j38z5tzkrw1ven95tglmszfwz0kqhuyuf.png)
Therefore, if the frequency is halved the period is doubled.