早教吧 育儿知识 作业答案 考试题库 百科 知识分享

设x+y=1,x2+y2=2,求x7+y7的值.

题目详情
设x+y=1,x2+y2=2,求x7+y7的值.
▼优质解答
答案和解析
∵x+y=1,x2+y2=2,
∴xy=-
1
2

∴x3+y3=(x+y)(x2+y2-xy)=1×(2+
1
2
)=
5
2

又∵(x4+y4)(x3+y3)=x7+y7+x3y3(x+y),
∴x7+y7=(x4+y4)(x3+y3)-x3y3(x+y)
=[(x2+y22-2x2y2](x3+y3)-x3y3(x+y)
=(22-2×(−
1
2
)2)×
5
2
-(−
1
2
)3×1
=
71
8