Can anyone help me explain why r is between 0 and 512 on Cal

Can anyone help me explain why r is between 0 and 5^(1/2) on Calculus Trans 6th Ed, Stewart Chapter 15.7 #19

07/07/12 -- Please rate my answer to your question: \"Calculus: Early Transcendentals 6th Edition Stewart 15.7 #19.\" at http://www.cramster.com/answers-may-12/calculus/calculus-early-transcendental-explain-0-51-2_2539683.aspx?rec=0