设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15=

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设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15=设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15=设等差数列an的前n项和为Sn,若S6=24,S9=63,则S1

设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15=
设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15=

设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15=
由S6=24得 a1+a6=8,由S9=63得 a1+a9=14和 a5=7
所以a1+a9-（a1+a6）=14-8,
所以a9-a6=6
所以公差d=6÷3=2
又因为 a5=7
所以a15=27,a1=-1.
所以S15=195

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