S6=4S3 6a1+15d=4(3a1+3d)6+15d=4(3+3d)d=2a4=a1+3d=7S4=5S2a1(1-q^4)/(1-q)=5a1(1-q^2)/(1-q)1+q^2=5q²=4Q＜1q=-2a3=2a1q²=2a1=1/2aN=a1q^(n-1)=1/2(-2)^(n-1)=(-2)^(n-2)第1问题也是等比数列。不能用公差d。a1(1-q^4)/(1-q)=5a1(1-q^2)/(1-q)1+q^2=5这里怎么化简来的。看不明白。a1=1,S6=4S3 则a4=a1(1-q^6)/(1-q)=4a1(1-q^3)/(1-q)1-q^6=4(1-q^3)(1-q³)(1+q³)=4(1-q^3)1+q³=4a4=a1q³=4a1(1-q^4)/(1-q)=5a1(1-q^2)/(1-q)同时乘以1-qa1(1-q^4)=5a1(1-q^2)同时除以a11-q^4=5(1-q^2)(1-q²)(1+q²)=5(1-q^2)1+q^2=5