若Sn为等差数列前n项和,且S8-S4=12,则S12的值为?_其他解答

若Sn为等差数列前n项和,且S8-S4=12,则S12的值为?

人气：386 ℃　时间：2020-06-03 13:48:35

解答

s8 - s4 =a5 + a6 + a7 + a8
a5 + a6 + a7 + a8 = 12
4a1 + 22d = 12
2a1 + 11 d = 6
a1 + (a1 + 11 d ) = 6
a1 + a12 = 6
s12 = 12 (a1 + a12 ) / 2 =12 X 6 /2 =36

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