“A8,A9,A11依次成等差数列”与An是等差数列矛盾.估计应该是“A8,A9,A11依次成等比数列”.
设公差为d A8=A1+7d=-12+7d A9=A1+8d=-12+8d A11=A1+10d=-12+10d
A8*A11=A9² (-12+7d)(-12+10d)=(-12+8d)² 144-204d+70d²=144-192d+64d²
6d²-12d=0 d=2 d=0(舍去)
An的公差为:2
“A8,A9,A11依次成等差数列”与An是等差数列矛盾.估计应该是“A8,A9,A11依次成等比数列”.
设公差为d A8=A1+7d=-12+7d A9=A1+8d=-12+8d A11=A1+10d=-12+10d
A8*A11=A9² (-12+7d)(-12+10d)=(-12+8d)² 144-204d+70d²=144-192d+64d²
6d²-12d=0 d=2 d=0(舍去)
An的公差为:2