1.
Sn=a1(qⁿ-1)/(q-1)
n=6 a1=3 q=2代入
S6=a1(q^6 -1)/(q-1)=3×(2^6 -1)/(2-1)=3×63=189
2.
Sn=a1(1-qⁿ)/(1-q)
n=5 a1=8 q=1/2代入
S5=8×(1-1/2^5)/(1-1/2)=8×(31/32)/(1/2)=31/2
3.
a1=1
q=a2/a1=2/1=2
a5+a6+...+a10=S10-S4
=1×(2^10 -1)/(2-1) -1×(2^4 -1)/(2-1)
=2^10-1-2^4+1
=2^10-2^4
=1024-16
=1008