Prove that the value of sqrt(a-1) is a rational number.

att:solvers

...Prove that the value of sqrt(a-1) is a rational number

was   sqrt(a)-1 in the original text, not sqrt(a-1)

Clearly if   sqrt(a)-1=m/n 

  then  sqrt(a)=1+m/n= (m+n)/n   ,   a rational number

Brian has actually discovered the error and solved  assuming 
"Prove that the value of sqrt(a) is a rational number"