[tex]\int\limits^4_0 {\sqrt{1+(y')^2} } \, dx =\int\limits^4_0 {\sqrt{1+9x/4} } \, dx =\\=1/2\int\limits^4_0 {\sqrt{(4+9x)} } \, dx[/tex]

L=1/2 (9x+4)^(3/2)*2/27 x=4 x=0

L=1/27((40)^(3/2)-4^(3/2))=8/27((√10)³-1)≈9,1