Understanding MSE formula for two variables

Hi DQ community,

From the Gradient Descent mission (screen 5), we can learn that the MSE(a0,a1)=(1/2n) * the sum of the (a0+a1x-y)². I am wondering why it is (1/2n) instead of (1/n) - as it is in the ‘simple’ MSE formula - since we still have n elements in the sum and the MSE is the mean of those n elements.

Thanks for your help,

Furthermore, as a0 approaches 0, we have (a0+a1x-y)² approaching (a1x-y)² but their respective MSE still differ by a factor of 2!!