∴q？ =8/(- 16)=- 1/2

∴a7=a 1q^6=(- 16)(- 1/2)？ =-4

2、∫a3+a4+a5 = 2 a4+a4 = 3 a4 = 12

∴a4=4

∴a 1+a2+...+a7 =(a 1+a7)+(a2+a6)+(a3+a5)+a4 = 2 a4+2 a4+2 a4+a4 = 7 a4 = 28

That is: a 1+a2+...+a7=28.

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