在等差数列{an}中,已知a8-a5=6,a10+a12=a20.求首项和公差
人气:129 ℃ 时间:2020-09-14 08:23:10
解答
a8 - a5 = 3d = 6 ==> d = 2
a10 = 9d + a1
a12 = 11d + a1
a20 = 19d + a1
2a1+ 20d = a1 + 19d ==> a1 = -d = -2
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