[题目]设 (x,y)=(int )_(0)^1f(t)|xy-t|dt, 其中f(t)在[0,1]上-|||-连续, leqslant xleqslant 1 ,0≤y≤1, 求 dfrac ({partial )^2u}(partial {x)^2}cdot dfrac ({partial )^2u}(partial {y)^2},