0=a*(3a＋4b＋5c)=3a²+4ab+5ac=3+4ab+5ac
0=b*(3a＋4b＋5c)=3ab+4b²+5bc=4+3ab+5bc
0=c*(3a＋4b＋5c)=3ac+4bc+5c²=3ac+4bc+5
解得：ab=0 bc=-4/5 ca=-3/5
a*(b+c)=-3/5