已知数列{an}为等差数列，a3=54，a7=-34，则a15的值为．

Question

已知数列{a_n}为等差数列，a₃=(5)/(4)，a₇=-(3)/(4)，则a₁₅的值为 ___ ．

Answer 1

等差数列{a_n}中，a₃=(5)/(4)，a₇=-(3)/(4)，设公差为d
则d=(a7-a3)/(7-3)=(-(3)/(4)-(5)/(4))/(4)=-(1)/(2)
∴a₁₅=a₇+8d=-(3)/(4)+8×（-(1)/(2)）=-(19)/(4)
故答案为：-(19)/(4)