[题目]设函数 u=f(x,y) 具有二阶连续偏导-|||-数,且满足等式 dfrac ({partial )^2u}(partial {x)^2}+12dfrac ({partial )^2u}(partial xpartial y)+5dfrac ({partial )^2u}(partial {y)^2}=0. 确定-|||-a,b的值,使等式在变换 =x+ay =x+by 下简-|||-化 dfrac ({a)^2u}(partial varepsilon partial n)=0.