已知数列{an}为等差数列,a3=54,a7=-34,则a15的值为______.-数学

题目简介

已知数列{an}为等差数列,a3=54,a7=-34,则a15的值为______.-数学

题目详情

已知数列{an}为等差数列,a3=
5
4
,a7=-
3
4
,则a15的值为______.
题型:填空题难度:偏易来源:不详

答案

等差数列{an}中,a3=class="stub"5
4
,a7=-class="stub"3
4
,设公差为d
则d=
a7-a3
7-3
=
-class="stub"3
4
-class="stub"5
4
4
=-class="stub"1
2

∴a15=a7+8d=-class="stub"3
4
+8×(-class="stub"1
2
)=-class="stub"19
4

故答案为:-class="stub"19
4

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