a=√(1+x)
x=3,a=2
x=0,a=1
x=a²-1
dx=2ada
所以原式=∫(1→2)2a(a²-1)da/a
=∫(1→2)2(a²-1)da
=2a³/3-2a(1→2)
=(16/3-4)-(2/3-2)
=8/3