等差数列{a小n}中,a1加a2加a3=9,a1乘a2乘a3=15,则a1=?a小n=?急
问题描述:
等差数列{a小n}中,a1加a2加a3=9,a1乘a2乘a3=15,则a1=?a小n=?急
答
由{a小n}为等差数列,设公差为d,
则a1+a2=a3=3a1+3d=9,即a2=3,
a1+a2+a3=a1+a3=6,a1*a3=5,可得方程:a1平方-6a1+5=0,a1=1或5
所以d=2,则{a小n}=2n-1或2n+3