Perimeter is the measure of the distance around a shape. The most basic way to calculate a perimeter is by adding up the length of all the sides. However, there are situations where you won’t be given the length of all the sides, or might just want to calculate the perimeter faster. In these situations you can use formulas to calculate perimeters of specific shapes. These formulas will be available to you during the GED math test.

Perimeter of a Square

Perimeter = 4 x side

When you need to find the perimeter of a square all you need to do is multiply one side by four.

Perimeter of a Rectangle

Perimeter = 2 x length + 2 x width

Multiply one of the short sides by two and multiply one of the long sides by, then add those two numbers together.

Perimeter of a Triangle

Perimeter = side1 + side2 + side3

If the triangle does not have a right angle, the way to find the perimeter is by adding up the length of all three sides.

3 + 4 + 5 = 12

If you need to find the perimeter of a triangle with a right angle, you can find the perimeter even if you only know the lengths of two of the sides.

The way you do that is by multiplying each side by using the Pythagorean Theorem (a² + b² = c ²).

Now you just add the tree sides together as above to get the perimeter.

5 + 4 + 6.4 = 15.4

Circumference of a Circle

Circumference = π x diameter

When you talk about the distance around a circle the term circumference is used instead of radius. The diameter is how wide a circle is at its widest point.

π is approximately 3.14, so just use 3.14 for π (pi).

If you are only given a circle’s radius in a question, you can still find the circumference by multiplying the radius by two before multiplying by π, since the radius is half the diameter.

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  • shelby

    How do you find the exact degrees of an angle?

  • shelby

    Never mind! I read triangle properties and there it was but thank you anyways! your examples are AMAZING!!:)

  • Jerrica Duncan

    An the exmaple of Pythagorean Theorem hw did you come up with the answer 41.

    • Ziyi

      The 41 comes from adding 25 and 16 together. The 25 comes from squaring (multiplying a number by itself) the length of one of the sides of the triangle that we know, 5. Five times 5 is 25. The 16 comes from squaring the other side we know, 4. I have some more about the Pythagorean theorem here.

  • Mika

    how did you get 6.4 as the square root of 41?

  • CatreshaP.

    I need to know how did you get 6.4 as the squareroot of 41 ?????? Very confused on this one!!

    • snu123

      h=sq.root of p^2+b^2
      where p=perpendicular,b=base
      h=hyptenuse(longest side)
      p=perpendicular(opp of the longest side)
      b= base (shortest line segment)

  • CatreshaP.

    I,m still coming up with 1681 as the squareroot of 41 on my casio-fx260!!!& I,ve been using this brand of calculator for a while.Please tell did I miss something? Or can you explain how you came up with the 6.4??? Need assistance ASAP! Thanks

    • Ziyi

      Hi, I think you’re confusing square roots and squares. When you find the square root of something you find what number times itself equals your number. When you square something you multiply that number by itself. For example the square root of 16 is 4 since 4 times 4 is 16. But 16 squared is 256 (16 time 16). On the casio-fx260 the square and square root share the same button. It’s in the middle of the top row of buttons and looks like x with a small two floating next to it. When you want to square a number, hit the number and that button. That’s what you did with 41. But if you want the square root you want to use the button’s function in yellow above the button. It looks like a check mark with a line comming out of it. To use the yellow function of a button hit shift then the button. So you put 41, shift (top left button), then the same button you used for squaring. You should get 6.4 something. I just rounded to 6.4 in the example above.

      • CatreshaP.

        Wow ! Was,nt thinking logically ! Thanks sooooooo much for everything ! 🙂 you,ve been of huge help

  • CatreshaP.

    Someone just told me that I need a scientific calculator to find the squareroot of 41,is this true?? From my understanding the casio-fx260 is as scientific as its gonna get!!! 🙂 🙂 help!!

    • Ziyi

      You don’t need to have a scientific calculator to find square roots, but scientific calculators can also be used to find the square root. Most basic calculators have the square root button. The casio-fx260 has it too, just hit 41 and the square root button.

  • Q

    In my GED 2013, every problem ask me to use ratios like 3:4:5 to solve if I don’t have any other variables to use its so confusing, Does Sin and cos better to use?

  • nick

    i got 6.5 can you show me what to do to find 6.4 rather than just the answer or using a calculator

  • ariel

    Now you just add the tree sides together as above to get the perimeter

  • Karima

    41 ( shift ) the second key from shift= 6.403124237 ( using the casio fx-260 )
    So, 5+4+6.4=15.4

  • Kisha

    O.M.G… this was so help, wonder why i didnt find this site sooner? Im taking the ged math 2morrow and so NERVOUS… but i have faith that i will pass now knowing what i know about these formulas… Thanks a Bunch, and Wish me LUCK!!!

  • Karla Diaz

    I really love this web!!!
    tomorrow I got my test and I hope to ge a good score! thanks… 🙂

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