I try to run multiple regression model. By plotting the model residuals against each continuous variable in my model, I see that there is non-linearity in some cases. I use poly function in R to raise these variables to 2nd power. I do it one by one and run anova to compare the models. When I add one of the polinomians in my model, the adjusted R2 does not change, although anova shows that adding polinomial improves model fit. Should I include polinomial in this case? Especially taking into account reduced degrees of freedom and more complicated interpretation. Also, I have 6 independent variables in my model. Two of them seems to be quadratic. Is it ok? I am absolutely new to quantitative analysis Thanks!