急设Sn为等比数列前n项和,a2=4,S2=2,（一）求an,（二）设等差数列bn前n项和为设Sn为等比数列前n项和,a2=4,S2=2,（一）求an,（二）设等差数列bn前n项和为Tn,若b2=a1,b5=a2,求bn和T8

Question

急设Sn为等比数列前n项和,a2=4,S2=2,（一）求an,（二）设等差数列bn前n项和为
设Sn为等比数列前n项和,a2=4,S2=2,（一）求an,（二）设等差数列bn前n项和为Tn,若b2=a1,b5=a2,求bn和T8

Answer 1

S2=a1+a2=a1+4=2
a1=-2
q=a2/a1=4/(-2)=-2
故an=a1q^(n-1)=-2*(-2)^(n-1)=(-2)^(n-1)
b2=a1=-2,b5=a2=4
d=(b5-b2)/3=(4+2)/3=2
b1=b2-d=-2-2=-4
故bn=b1+(n-1)d=-4+(n-1)*2=2n-6.
T8=8b1+8*(8-1)*d/2=8*(-4)+28*2=24