11×4+14×7+17×10+…+1(3n-2)(3n+1)=()A.n3n+1B.n+13n+1C.2n-13n+1D.2n-23n+1-数学

题目简介

11×4+14×7+17×10+…+1(3n-2)(3n+1)=()A.n3n+1B.n+13n+1C.2n-13n+1D.2n-23n+1-数学

题目详情

1
1×4
+
1
4×7
+
1
7×10
+…+
1
(3n-2)(3n+1)
=(  )
A.
n
3n+1
B.
n+1
3n+1
C.
2n-1
3n+1
D.
2n-2
3n+1
题型:单选题难度:偏易来源:不详

答案

原式=class="stub"1
3
(1-class="stub"1
4
)+class="stub"1
3
class="stub"1
4
-class="stub"1
7
)+…+class="stub"1
3
class="stub"1
3n-2
-class="stub"1
3n+1
)=class="stub"1
3
[(1-class="stub"1
4
)+(class="stub"1
4
-class="stub"1
7
)+…+(class="stub"1
3n-2
-class="stub"1
3n+1
)]=class="stub"1
3
(1-class="stub"1
3n+1
)=class="stub"n
3n+1

故选A.

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