等差数列求解A4=10且A3,A6,A10成等比数列且公比d不等于零.求等差数列s20

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等差数列求解A4=10且A3,A6,A10成等比数列且公比d不等于零.求等差数列s20
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等差数列求解A4=10且A3,A6,A10成等比数列且公比d不等于零.求等差数列s20
等差数列求解
A4=10且A3,A6,A10成等比数列且公比d不等于零.求等差数列s20

等差数列求解A4=10且A3,A6,A10成等比数列且公比d不等于零.求等差数列s20
看图不说话:

A3 A6 A10成等比数列 A6^2=A3*A10 (A4+2d)^2=(A4-d)(A4+6d) A4=10 d=1 An=7+(n-1)d Sn=n(n-1)d/2 S20=190

令等差数列的公差为X
A6*A6=A3*A10 (A4+2X)^2=(A4-X)(A4+6X)
解得A4=10X,
由A4=10得X=1
An=(10-4)+n*X=6+n
Sn=6n+n(n-1)/2=n(n+11)/2
S20=20*(20+11)/2=310