The error in the TP review lecture

So in 28 part (f), I forgot an "a" when I copied the formula for g' into the denominator in the expression for dx₀/da. The derivative is - as you find in the solution - as this picture, the missing "a" indicated in red. When taking the limit, that kills the last "1", as indicated with red in this picture. That results in 1/2, as it should. 

Thanks a lot to the student who found the error! Nils.