find the sum of the geometric series using a formula 1-4 16-64 256-1024 Post author By Sandy Spencer Post date Questions › Category: Maths › find the sum of the geometric series using a formula 1-4 16-64 256-1024 0 Vote Up Vote Down Sandy Spencer asked 9 months ago find the sum of the geometric series using a formula 1-4+16-64+256-1024 1 Answers 0 Vote Up Vote Down Sandy Spencer answered 9 months ago a = 1 , r = -4 you are showing 6 terms, so sum(6) = a(r^6 – 1)/(r-1) = 1((-4)^6 – 1)/(-4-1) = 4095/-5 = -819 Please login or Register to submit your answer Username or Email Address Password Remember Me ← the sum of anumber x and twice another is 20.if the product of these numbers is not → In an exam a student scored 60 marks out of a possible 80. What percentage is this