87
$\begingroup$

Just curious about why geometric progression is called so. Is it related to geometry?

$\endgroup$
84
$\begingroup$

Here is a geometric figure illustrating the geometric progression $1,r,r^2,r^3,r^4,r^5,\ldots$:

enter image description here

$\endgroup$
  • 4
    $\begingroup$ Now that's a good picture! We don't have to worry about whether people were thinking about $8$-dimensional cubes, however long ago the terminology came about. $\endgroup$ – pjs36 May 14 '15 at 14:26
  • 2
    $\begingroup$ So I guess we pick $r$ and draw the edge you have labelled as $r$. Then the hypotenuse of the resulting triangle is $r^2$? Can you please post a proof of that? Does this depend on the length of the other two edges of the first right triangle (the leftmost one with one leg labelled $1$). You haven't labelled them and they aren't uniquely determined by one leg being length one. Thank you. $\endgroup$ – Gregory Grant May 14 '15 at 14:37
  • 5
    $\begingroup$ @GregoryGrant The triangles are all similar, that uniquely determines everything. The second triangle from the bottom is drawn first and then the first triangle is drawn similar to the second and so on... $\endgroup$ – Asvin May 14 '15 at 14:47
  • 3
    $\begingroup$ Is there any useful information that can be read off this figure? What would it look like for the interesting case when $r < 1$? $\endgroup$ – Rob Arthan May 14 '15 at 19:16
  • 8
    $\begingroup$ @RobArthan If $r<1$, you draw the figure approaching the vertex rather than going away from it. $\endgroup$ – Mario Carneiro May 15 '15 at 9:13
88
$\begingroup$

Here's an extremely nice picture illustrating the geometric series and making it geometrically clear how it converges when $x<1$.

Dobbs 1918

$s-1=xs$ follows from $PN/ON=BA/OA$ which follows from the fact that $\triangle OAB$ is similar to $\triangle ONP$.

$\endgroup$
  • 2
    $\begingroup$ Great added bonus for the (unfamiliar to me) proof for the sum :) $\endgroup$ – nbubis May 15 '15 at 4:11
  • $\begingroup$ @nbubis Right, I only knew how to make the geometric series intuitive and visual for $x=1/2$, but this allows one to do it for all $x\in(0,1)$. $\endgroup$ – Gregory Grant May 15 '15 at 17:47
  • 6
    $\begingroup$ What book is this? $\endgroup$ – Alec Teal May 16 '15 at 18:13
  • $\begingroup$ @AlecTeal It's this one: amazon.com/Plane-Trigonometry-H-S-Carslaw/dp/B00BGGHADM/… $\endgroup$ – Gregory Grant Jun 7 '15 at 12:51
  • $\begingroup$ @AlecTeal I realized you probably meant what book is the one that is referencing Carslaw. I'm not sure, I found it here: imgkid.com/geometric-sequences-formulas.shtml And I can't seem to find any other reference to it. It's pretty obscure I guess. $\endgroup$ – Gregory Grant Jun 7 '15 at 14:05
12
$\begingroup$

Let $ABC$ is a right triangle with right angle $\angle ABC$, then if we draw the height $BH$, we have $$ |BH|^2=|AH||CH| $$ The geometric mean comes from here...

$\endgroup$
  • 1
    $\begingroup$ Isn't the height of the right triangle just $|BC|$? $\endgroup$ – Gregory Grant May 14 '15 at 13:10
  • 2
    $\begingroup$ Observe that the right angle is $\angle B$ $\endgroup$ – k1.M May 14 '15 at 13:11
  • 1
    $\begingroup$ Ah, so you mean drop a perpendicular from the point $B$ to its opposite side, then $H$ is the point of intersection? $\endgroup$ – Gregory Grant May 14 '15 at 13:13
  • 1
    $\begingroup$ Yes...Exactly... $\endgroup$ – k1.M May 14 '15 at 13:14
4
$\begingroup$

My guess would be that geometric sequences arose as generalization of sequence $a,a^2,a^3,...$. Why is this geometric? Well, $a$ is the length (= 1-dimensional "volume") of line segment (= 1-dimensional hypercube) of "side" length $a$, $a^2$ is the area (= 2-dimensional "volume") of a square (= 2-dimensional hypercube) of side length $a$, $a^3$ is the volume (= 3-dimensional "volume") of a cube (= 3-dimensional hypercube) of side length $a$ and so on.

$\endgroup$
3
$\begingroup$

The arithmetic and geometric adjectives come from the Pythagoreans before the Christian Era. Apparently, the expression “geometric progression” comes from the “geometric mean” (Euclidean notion) of segments of length $a$ and $b$: it is the length of the side $c$ of a square whose area is equal to the area of the rectangle of sides $a$ and $b$. The construction of the geometric mean with ruler and compass is well known for high school students; it involves "multiplication" and not "addition".

enter image description here

$\endgroup$
1
$\begingroup$

Because geometric progressions are based on multiplication, and the most important geometric notion, namely, volume, arises from multiplication (length times width times height). The term “multiplicative” is not used because it already has a special meaning in Number Theory.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.