Worksheet: Sum of a Finite Geometric Sequence

In this worksheet, we will practice calculating the sum of the terms in a geometric sequence with a definite number of terms.

Q1:

The sum of the terms of a sequence is called a series.

A geometric series is the sum of a geometric sequence; a geometric series with 𝑛 terms can be written as 𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟,() where 𝑎 is the first term and 𝑟 is the common ratio (the number you multiply one term by to get the next term in the sequence, 𝑟1).

Find the sum of the first 6 terms of a geometric series with 𝑎=24 and 𝑟=12.

  • A812
  • B4714
  • C2314
  • D204
  • E1158

Q2:

A geometric series has a first term of 3 and a common ratio of 5. Find the sum of the first 6 terms.

Q3:

Find, to two decimal places, the sum of the following geometric series: 20+20(1.01)+20(1.01)++20(1.01).

Q4:

We can find a formula for the sum of a geometric series. Consider the series 𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟.

Multiply the expression for 𝑆 by 𝑟, the common ratio.

  • A𝑟𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟
  • B𝑟𝑆=𝑎𝑟+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟
  • C𝑟𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟
  • D𝑟𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟
  • E𝑟𝑆=𝑎𝑟+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟

So, we have 𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟 and 𝑟𝑆=𝑎𝑟+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟.

The right-hand sides of the equations are very similar. Identify the terms that do \textbf{not} appear on the right-hand side of both equations.

  • A𝑎𝑟,𝑎𝑟
  • B𝑎,𝑎𝑟
  • C𝑎,𝑎𝑟
  • D𝑎,𝑎𝑟
  • E𝑎𝑟,𝑎𝑟

Now, consider the subtraction 𝑆𝑟𝑆=𝑎+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟𝑎𝑟+𝑎𝑟+𝑎𝑟+𝑎𝑟++𝑎𝑟.

Use the answer to the previous part to simplify the subtraction 𝑆𝑟𝑆.

  • A𝑆𝑟𝑆=𝑎𝑟𝑎𝑟
  • B𝑆𝑟𝑆=𝑎𝑎𝑟
  • C𝑆𝑟𝑆=𝑎𝑟𝑎𝑟
  • D𝑆𝑟𝑆=𝑎𝑎𝑟
  • E𝑆𝑟𝑆=𝑎𝑎𝑟

Factor both sides of the equation.

  • A𝑆(1𝑟)=𝑎𝑟(1𝑟)
  • B𝑆(1𝑟)=𝑎1𝑟
  • C𝑆(1𝑟)=𝑎𝑟𝑟
  • D𝑆(1𝑟)=𝑎1𝑟
  • E𝑆(1𝑟)=𝑎(1𝑟)

Rearrange the equation to make 𝑆 the subject of the formula.

  • A𝑆=𝑎𝑟(1𝑟)1𝑟
  • B𝑆=𝑎1𝑟1𝑟
  • C𝑆=𝑎1𝑟1𝑟
  • D𝑆=𝑎𝑟𝑟1𝑟
  • E𝑆=𝑎(1𝑟)1𝑟

Q5:

There are two geometric series with a first term of 3 and whose first 3 terms have a sum of 21.

What are their common ratios?

  • A3, 7
  • B2, 3
  • C2, 3
  • D2, 3
  • E3, 7

Write an expression for the sum of the first 𝑛 terms of the sequence with first term 3 and common ratio 2.

  • A321
  • B32
  • C2(13)
  • D23
  • E231

Q6:

In a geometric sequence, the first term is 𝑎 and the common ratio is 𝑟.

Find the sum of the first 3 terms of a geometric sequence with 𝑎=328 and 𝑟=14.

  • A3692
  • B410
  • C5332
  • D8612

Q7:

Find the geometric sequence given the first term is 324, the last term is 4, and the sum of all the terms is 484.

  • A324,972,2,916,,4
  • B324,972,2,916,,4
  • C324,108,36,,4
  • D324,108,36,,4

Q8:

Find the fourth term of the geometric sequence given by 𝑆=1,0242 where 𝑆 is the sum of the first 𝑛 terms.

Q9:

Find the first three positive terms of a geometric sequence given their sum is 14 and the sum of their multiplicative inverses is 6332.

  • A32, 6, and 24
  • B32, 38, and 332
  • C23, 83, and 323
  • D23, 16, and 124

Q10:

Find, in terms of 𝑙 and 𝑚, the common ratio of the geometric sequence given the sum of the first ten terms is 24𝑙 and the sum of the next ten terms is 91𝑚.

  • A24𝑙91𝑚
  • B24𝑙91𝑚
  • C91𝑚24𝑙
  • D24𝑚91𝑙
  • E91𝑚24𝑙

Q11:

Find the geometric sequence and the sum of the first six terms given the second term is four times the fourth one, the sum of the fourth and seventh terms is 2 and all terms are positive.

  • A𝑎=1289,649,329,, 𝑆=28
  • B𝑎=136,118,19,, 𝑆=3136
  • C𝑎=2569,1289,649,, 𝑆=56
  • D𝑎=136,118,19,, 𝑆=74
  • E𝑎=1289,649,329,, 𝑆=2489

Q12:

Find the sum of the first 20 terms of the geometric sequence 1,1.07,1.07,1.07, giving the answer to two decimal places.

Q13:

Find the sum of the geometric series 322122+16611,782.

  • A91891
  • B8722
  • C61594
  • D24911

Q14:

Find the sum of the geometric series 176+88+44++11.

Q15:

Find the geometric sequence and the sum of the first six terms given that the sum of the second and fourth terms is 68 and the sum of the third and fifth terms is 272.

  • A𝑎=(256,64,16,), 𝑆=341
  • B𝑎=14,1,4,, 𝑆=1,3654
  • C𝑎=(1,4,16,), 𝑆=341
  • D𝑎=(1,4,16,), 𝑆=1,365
  • E𝑎=(256,64,16,), 𝑆=1,3654

Q16:

Find the sum of the first 7 terms of a geometric sequence given 𝑎=8𝑎 and 𝑎+𝑎=64.

  • A1,3765
  • B1685
  • C3445
  • D1,3445
  • E1725

Q17:

Find the number of terms of the geometric sequence 𝑎=96×2 for which the sum is equal to 93.

Q18:

Find the least number of terms of the geometric sequence given the first term is 1 and the fourth term is 125, where the sum of the terms is 7,143.

Q19:

In a geometric sequence, the first term is 𝑎, the common ratio is 𝑟, and the last term is 𝑙.

Find the sum of the geometric sequence with 𝑎=1,408, 𝑟=12, and 𝑙=88.

Q20:

Find the geometric sequence and the sum of the first seven terms given 15𝑎6𝑎=9𝑎, 𝑎=9 and all terms are positive.

  • A𝑎=127,19,13,, 𝑆=1,09327
  • B𝑎=(288,144,72,), 𝑆=567
  • C𝑎=181,127,19,, 𝑆=1,09381
  • D𝑎=(288,144,72,), 𝑆=1,1432
  • E𝑎=127,19,13,, 𝑆=36427

Q21:

Find the sum of the first 6 terms of the geometric series 12+14+18+116+.

  • A3112
  • B3132
  • C1512
  • D6364

Q22:

Find the geometric sequence and the sum of the first five terms given the sum of the first three terms is 1 and the sum of the next three terms is 27.

  • A𝑎=913,313,113,, 𝑆=4039
  • B𝑎=113,313,913,, 𝑆=12113
  • C𝑎=113,313,913,, 𝑆=4013
  • D𝑎=913,313,113,, 𝑆=121117
  • E𝑎=2713,8113,24313,, 𝑆=3,26713

Q23:

Find the number of terms of the geometric sequence (23,69,207,) for which the sum is equal to 2,783.

Q24:

Find the sum of the first 6 terms of the geometric sequence (405,135,45,).

  • A1603
  • B1,8203
  • C1,8256
  • D605

Q25:

Find the sum of the geometric sequence (16,32,64,,256).

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