The common ratio of a geometric sequence is the fixed number that each number in the sequence, except the first number or starting number, is multiplied by. The figure below shows the common ratio for the geometric sequence 2, 6, 18, 54, ...
Notice that 6 / 2 = 3, 18 / 6 = 3, and 54 / 18 = 3. The name common ratio came from the fact that the ratio between consecutive numbers is constant.
a.
Geometric sequence: 5, 20, 80, 320, ...
The common ratio is 4
b.
Geometric sequence: 4, 20, 100, 500, ...
The common ratio is 5
c.
Geometric sequence: 4, 4r, 4r^{2}, 4r^{3}, ...
The common ratio is r
There is something interesting about c.!
The second number is 4r = 4r^{1}
The third number is 4r^{2}
The fourth number is 4r^{3}
What can we conclude about the nth number?
Notice that the exponent shown in blue is always one less than the term number (shown in red). For example, for 4r^{3}, the exponent is 3 for the 4th number.
We can also say that for 4r^{n-1}, the exponent is n-1 for the nth.
Therefore, nth term = 4r^{n-1 }for the geometric sequence 4, 4r, 4r^{2}, 4r^{3}, ...
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