Those Handy Maths Formulars

I don’t do math. I do AutoCAD. :-)

-- Gary, East TX -- The longest journey begins with a single step.

A golden rectangle has sides with a ratio close to 5:8. If you want be exact make it 1:(1+sqrt(5))/2

Supposedly, this is the most eye pleasing rectangle.

-- Scott, San Diego

My favorite is (and always will be): 8 + 2 = 10. Then (and ONLY then), 5:00 + 10 = Miller Time!

Here’s one that took a bit of research. Find the angle between two faces of any regular pyramid.

cos(A) = - ((R^2 + (2 x H^2+R^2) x cos(t))/(2 x H^2 + R^2 + R^2 x cos(t)))

Find the arccosine and you’re done!

Note: I had to play with the formatting a bit, to get the formula to display correctly. LJ kept ‘interpreting’ my multiplication asterisks as formatting commands, which changes parts of the formula to BOLD, AND leaves out the asterisks. Does anybody know how to suppress the auto formatting, temporarily?

Anyway,

A= the angle between the faces (the number I needed)

and I knew

R= the distance from the center of the pyramid’s base, to the vertices
H= the height of the pyramid
t= the angle (in deg) between the bases of adjacent pyramid faces (t=360/n where n= the number of faces – e.g. for a hexagonal pyramid, t=60 deg)

This works for any ‘regular’ pyramid, regardless of size, or number of faces—think gazebo roof. I got tired of punching the numbers into a calculator, so I put the formula into an Excel spreadsheet. Enter those three pieces of info, and it calculates the rest.

If you’d like a copy of that Excel file, let me know.

The formula came from, http://mathforum.org/library/drmath/view/55203.html. They explain this more better than I can.

-- There is nothing in the world more dangerous, than a woodworker who knows how to read a micrometer...

I just usually consult one of the numerous math related links I’ve saved. If I cant find what I need there, then I dust off my rusty trig/calculus and commence to cyphering with my gazintas ala Jethro.
http://www.intmath.com/
http://www.csgnetwork.com/sawmitercalc.html
http://www.woodweb.com/knowledge_base/Radius_of_Convex_Wall.html
http://johnmcgaw.com/wwsoft.html
http://www.jsommer.com/WoodMath/WoodMath.html
http://www.ajdesigner.com/phptriangle/equilateral_triangle_inscribed_circle_radius_r.php

-- Use the fence Luke

I can never remember how to calculate the dimensions of a jig to use with a router template. That is, how much bigger to make the outline to be cutout.

It’s something like the the difference between the outside diameter of the template ring and the bit, halved?

Here is a website that has a calculator for almost anything you have to crunch numbers for.

http://www.martindalecenter.com/Calculators1A_5_Co_RZ.html#HOMER-WOODW-BOOKSHELF
This is just the wood calculator section, there are over 23,465 calculators available!

Then if you go to the main reference page, there are thousands of other references.

http://www.martindalecenter.com/

-- Jim

Opps;

I think I’m on the wrong site!

Lee

-- by Lee A. Jesberger http://www.prowoodworkingtips.com http://www.ezee-feed.com

Son:” Dad Pi r square”
Dad: ” No son Pi are round cornbread r square”

Seriously folks I have marked this thread as a favorite. Thanks

-- Growing older but not up!

Tremendous Jocks & great links as well. what about the simple ones that the tradesmen use to take short cuts. there must be heaps out there.

-- Grumpy - "Always look on the bright side of life"- Monty Python

I do appreciate the links, since I’m not an advanced math person. I think it goes back to high school, where I had a geometry teacher that we nicknamed Froggy. One day I brought in a picture of a frog with the teacher’s face cut out of the yearbook and pasted over the frog’s head. Yup….I got busted with it.

On one site I saw this inetresting question – “Why Study Analytic Trigonometry? “

I can’t think of any reason at all…....