Those Handy Maths Formulars

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Forum topic by Grumpy posted 04-13-2008 04:14 AM 6333 views 8 times favorited 27 replies Add to Favorites Watch
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23836 posts in 3813 days

04-13-2008 04:14 AM

Topic tags/keywords: maths problems maths calculations

I will start this one off Jocks. Partridge posted a forum topic on a maths problem
Following I thought it would be a good idea to start a forum on maths formulars and calculations for the benefit of all Jocks.
Many of us know the 3-4-5 rule for triangles which comes in very handy in woodwork. gator9t9 raised this one.You can construct a triangle of any size in that 3-4-5 proportion and it will be accurate.
I will start with the problem of finding the area of an irregular triangle when you know the lengh of the 3 sides but not the angles.

Area of Triangle
Area = √[s(s-a)(s-b)(s-c)]
where s=1/2 the perimeter & a,b&c are the lengths of the 3 sides.
Ok Jocks, lets see what else you have stored in the memory cells.

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

27 replies so far

View GaryK's profile


10262 posts in 3951 days

#1 posted 04-13-2008 06:27 AM

I don’t do math. I do AutoCAD. :-)

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

View swied's profile


74 posts in 3724 days

#2 posted 04-13-2008 07:06 AM

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

View TroutGuy's profile


224 posts in 3674 days

#3 posted 04-13-2008 08:40 AM

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?


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, 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...

View Doug S.'s profile

Doug S.

295 posts in 3670 days

#4 posted 04-13-2008 12:57 PM

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.

-- Use the fence Luke

View robdew's profile


86 posts in 3677 days

#5 posted 04-13-2008 03:45 PM

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?

View Suz's profile


51 posts in 3719 days

#6 posted 04-13-2008 04:17 PM

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

-- Jim

View Lee A. Jesberger's profile

Lee A. Jesberger

6851 posts in 3942 days

#7 posted 04-13-2008 04:29 PM


I think I’m on the wrong site!


-- by Lee A. Jesberger

View Toolz's profile


1004 posts in 3705 days

#8 posted 04-13-2008 06:30 PM

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

-- Larry "Work like a Captain but Play like a Pirate!"

View Grumpy's profile


23836 posts in 3813 days

#9 posted 04-14-2008 12:51 AM

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

View Boardman's profile


157 posts in 3724 days

#10 posted 04-14-2008 01:32 AM

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…....

View SDL683's profile


1 post in 2378 days

#11 posted 10-16-2011 08:49 AM

heres a math question.
i suck at math… so heres a question i have.
If i take a 90 degree square surface thats 2” square. and i want to cut one corner flat to 3/4” wide
how far back on the sides do i make my mark???

-- Silas - Southern Ohio

View Eli's profile


141 posts in 2969 days

#12 posted 10-16-2011 03:19 PM

The radius of a circle is the same length as 1 side of an inscribed hexagon (one where all the points are on the circle). So if you set a compass to the radius, and walk it around the outside of the circle, it will break it into a hexagon.

Also, if you draw an inscribed right (all the points on the circle, one corner is 90 degrees) triangle in a circle, the hypotenuse (long side) always crosses through the center of the circle. Either find the center of the hypotenuse to find the center of the circle, or draw two of them and the lines will cross in the center.

The angles of a triangle always add up to 180 degress. You can take a polygon (a closed shape with some number of straight edges) and break it into triangles, then multiply the number of triangles by 180 and it will tell you how many degrees are in your shape. If your shape is regular (all of the angles are the same) then you can divide the degrees by the number of corners and it tells you what angle all of the corners are.


View CharlieM1958's profile


16274 posts in 4181 days

#13 posted 10-16-2011 03:32 PM

Silas: I think the correct formula would be A sq. X B sq. = C sq., with C = 3/4”.

But if you’ve got a combination square, who needs all that math? Just slide the 45 degree side of the square along one edge until the distance across the graduated side is 3/4”.

-- Charlie M. "Woodworking - patience = firewood"

View MrRon's profile


4710 posts in 3206 days

#14 posted 10-16-2011 06:02 PM

I remember when I went to school, way way back, kids would say; “all this math stuff won’t do me any good later in life”. I’m not a mathematician, but I’m glad I paid attention in class. I still do most math in my head, only using a calculator when working in decimals or converting English/Metric. I now realize how important a subject like Mathematics is, not just for the mathematicians, but for everyday use for you and me. Those of you with school age children; impress upon them how important it is to know their math; language also.

View Bill White's profile

Bill White

4901 posts in 3923 days

#15 posted 10-16-2011 06:34 PM

You’d be amazed at the number of young folks who can’t even read a tape measure. Scary!!!!


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