# Question regarding the R-squared of regressors in a multiple regression [duplicate]

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What is the R-squared of the regression of $X_2$ on $X_1$ if the R-squared of the regression of $X_1$ on $X_2$ is $R^2_1=0.5$? (where $X_1$ and $X_2$ are the two regressors in the regression $Y=\beta_0+\beta_1X_1+\beta_2X_2+u$)?