Question

已知遞增的等差數列{a_n}滿足a₂+a₃+a₄=15,?a₂a₃a₄=105,則a₁等于

A.1                   B.2                  C.3                   D.5

Answer 1

答案：A

解析：∵{a_n}是遞增的等差數列,a₂+a₃+a₄=15,a₂a₃a₄=105,

∴3a₃=15,a₃=5,a₂a₄=(a₃-d)(a₃+d)=a₃²-d²=21,

d²=25-21=4,d=2.∴a₁=a₃-2d=1.