Correlation between the estimated regression coefficients and correlation between the explanatory variables
Hello, What is the difference between the "correlation between the estimated regression coefficients (corr(b1, b2))" and the "correlation between the explanatory variables (corr(X1, X2))"? I found that, for example, corr(X1, X2) could be positive while corr(b1, b2) is negative. I think I understand corr(X1, X2), but I don't understand why we need to know corr(b1, b2), how it relates to corr(X1, X2...