等差数列{a n }中,S10=15则a 1 a 4 a 7 a 10等于多少?
等差数列{a n }中,S10=15则a 1 a 4 a 7 a 10等于多少?
数学人气:978 ℃时间:2020-02-03 06:10:40
优质解答
对于等差数列{ a n },设差为d,则:a n=a1+(n-1)d,Sn=n*a1+n*(n-1)*d/2那么:S10=10*a1+10*(10-1)*d/2=10a1+45d (1式)a1=a1;a4=a1+3d;a7=a1+6d;a10=a1+9d而a1+a4+a7+a10=a1+a1+3d+a1+6d+a1+9d=4a1+18d (2式)...
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