已知递减的等差数列an满足a1²=a9²,则a5=

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已知递减的等差数列an满足a1²=a9²,则a5=

递减的等差数列an满足a1²=a9²
则a1大于a9,a9=-a1
a9=a1+8*d
a1+a9=2a1+8d=2a5=a1+4d=0