已知:a+b+c=0,则a(1/b+1/c)+b(1/c+1/a)+c(1/a+1/b)+3的值是 _.
问题描述:
已知:a+b+c=0,则a(
+1 b
)+b(1 c
+1 c
)+c(1 a
+1 a
)+3的值是 ______. 1 b
答
若a+b+c=0,则a+b=-c,b+c=-a,a+c=-b
a(
+1 b
)+b(1 c
+1 c
)+c(1 a
+1 a
)1 b
=a•
+b•b+c bc
+c•a+c ac
a+b ab =a•
+b•−a bc
+c•−b ac
−c ab =−
a3+b3+c3
abc
∵a3+b3+c3-3abc=(a+b+c)(a2+b2+c2-ab-ac-bc)
∴当a+b+c=0时,a3+b3+c3-3abc=0
∴a3+b3+c3=3abc
∴原式=-3+3=0,
故答案为0.