A trend projection is a method used to predict future sales figures based on the sales figures from the previous period. The idea behind this method is that if a company has been experiencing consistent sales growth over a period of time, then it is likely that this trend will continue in the future.

To use the trend projection method, we need to first establish the baseline sales figure for the period in question. In this case, the baseline sales figure for December is $1,200.$ We then need to find the average sales figure for the previous three months $($October$,$ November, and December$),$ which is $1,100.$ Finally, we can project the sales for January by adding the average of the previous three months to the baseline sales figure:

January sales $[=1,100+1,200=2,300]$

Therefore, using the trend projection method, the forecast for the next month $($January$)$ is $2,300$ sales.

The trend projection method is a simple and effective way of predicting future sales figures, especially when the sales figures are relatively stable. However, it is important to note that this method assumes that the trend of increasing sales will continue, and so it may not be suitable for predicting sales figures for a company that has experienced a recent change in circumstances, such as a new product launch or a change in management.

Final answer: In conclusion, the trend projection method is a useful tool for predicting future sales figures, especially when the sales figures are relatively stable. However, it is important to note that this method assumes that the trend of increasing sales will continue, and so it may not be suitable for predicting sales figures for a company that has experienced a recent change in circumstances.