Multivariate Linear Regression with exponential trend-line The dataset that I'm working with has 2 independent variables (qty, volume) and 1 dependent variable (cost).  When I plot individual X with Y, it turns out qty vs cost gives an exponential decay trend line while volume vs cost gives a linear relationship.
I'm trying to come up with a linear model (since it's beginner-friendly) to predict the cost when a new input, volume set is given.  I tried to excel and while the statistic output looks reasonable, the predicted values are off quite a lot.  I read up on some articles and there is something regarding log transformation so that both qty and volume have a linear relationship with cost.  I'm not sure if log transformation applicable in this situation.  What would be the best way to approach this problem?
I plan to use R instead of excel next.
Here are some outputs of what I've done




Note: Thickness, width, and height essentially become volume so it doesn't affect the data much
I would suggest transforming &quot;qty&quot; into the log space. Even, you can do this using Excel. You can make a new column (for example qty_log), which is equal to the log of &quot;qty&quot;. Then, you can fit a linear regression as follows:
$cost = a1 * log(qty) + a_2 * volume + a_0$
You will get much better results in this case.
This equation simply means that try to fit a model using volume and log of qty for predicting cost. You did so before but you used qty instead of log(qty)