原式=(x-4)/(x-1)÷(x²-1-15)/(x-1)
=(x-4)/(x-1)÷(x²-16)/(x-1)
=(x-4)/(x-1)*(x-1)/(x+4)(x-4)
=1/(x+4)
=1/(5√2-4+4)
=√2/10