Can some 1 please help wid d below :

As per Eqn (10) on Page 311 Book 1 (CFAI) , T stat = (b1^ - b1 ) / sb^1

and Solution to EOR Question 5 (Page 341 book 1) says , T stat = [r*((n-2)^.5)]/[(1-r^2)^.5]

How do we know when to use which formula for calculating T stat ?

Thanks