 about perimeter. What is a perimeter? A perimeter or a circumference? A perimeter is the distance around the edges or the boundaries of a figure, so the outside part and we are able to calculate the length of this boundary and we can calculate the distance which is also measured by the length. Remember when we started with the SI system we talked about the length is always calculated with meters but you can convert from meters to centimeters and so forth. We use the length most of the time to calculate the distance between the boundaries around the boundaries. So we can also use simplification of calculations of the figure by looking at the properties of each. So it's very simple to calculate in a sense. It's very simple to calculate the perimeter because then the perimeter we're looking at the outside. So we just add all the sides together. So we can calculate the perimeter of a rectangle, a square, a triangle and a circle and we're going to follow each one of them and see how we can calculate the perimeter of each and every one of them. First rectangle is made up of four sides with two equal sides. So there are two sides that are equal at the top and at the bottom and also on the sides on the left and the right the sides will be equal and the top will be your length and your width will be on your side and we use the length and the width to calculate the circumference or the perimeter of this rectangle. So it means we're going to add all the sides. So on this one you must remember that this is the length is equal to this length the side. This is the width is equal to this width the side. To calculate the perimeter of a rectangle we use the formula c is equals to 2 times l plus 2 because there are two l's and there are two widths. Let's look at an example. Calculate the perimeter of a rectangle with the length of 30 and the width of 8. So it means this side is 8 and this side is 8 and this side is 30 and that side is 30 as well. Then we're going to substitute so our length 2 times our length plus width our length of 30 plus our width of 8 will give us 38 and 2 will give us 76 and therefore our perimeter of this rectangle is equals to 76 meters. We look at the square. A square has four equal sides so it means all the sides are equal. So it means if this is a square this side is equals to that side is equals to that side is equals to that side. The perimeter of a square the formula is 4 times l because there are four sides that are equal. Calculate the perimeter of a square with the length of 80 so therefore it means this is 80 that is 80 that is 80 and there is 80 so you can add 80 plus 80 plus 8. It will give you the perimeter or the circumference. c is equals to l times 4 which l is 80 times 4 it gives us 320 therefore the perimeter is 320 centimeter. Triangle has three sides and if we look at this let's say our sides are p, q, r those are the sides of a triangle. To calculate the perimeter we can add p plus r plus q and for example calculate the perimeter of a triangle with sides 10, 7 and 6 centimeters so it means we're going to add 10 plus 7 plus 6. We substitute into our formula 10 plus 7 plus 6 because p is you cannot miss 10 and q is 7 and r is 6. c will be equals to 23 therefore the perimeter of this triangle will be equals to 23 in the cycle so the second works totally different to the rectangle the square and the triangle. Why? Because the circle is a rounded figure and to calculate the perimeter of that circle we're going to be using what we call the circumference of the circle which is the rounded thing the inside. So how do we calculate this? If we look at a circle this is our circle it's round it has a point from this side to that side and that we call it the diameter. So the diameter starts from there to there in the middle and the midpoint if we stop right there and we go this way that is what we call the radius or even if we go that way is the radius as long as it's in the middle so from the middle to the outer side we call this side the radius and that is the radius and that is the radius. The red line represents your diameter your diameter is from one side of the circle to the other side of the circle. When we calculate the circle when we calculate the circle we need to know what our diameter is and what our radius is. So our diameter is made up of two radiuses so if we are here this is a radius one and radius two they add up to be its diameter. So to calculate the circumference of a circle or the perimeter of a circle we use the formula two pi r. Now on your calculators you must go and look for this sign on your calculator. The sign pi r you must go and look for this sign on your calculator. Sometimes if you are using a financial calculator it's written in orange so you'll have to press second function and then press that side. Do not use 22 over 7. Do not use that to calculate your pi because then you're going to cut off some of the digit. We're going to use this sign on your calculator called the pi sign okay. Let's look at how we calculate this. So to calculate the circumference we use two pi r and our r is our radius so let's look at an example. Calculate the circumference of a circle with the radius four. So to calculate with the radius of four. Now remember the radius it is from the midpoint to the outer side so we do not have to calculate or divide the diameter by two because our radius is already four we just substitute into that formula. So how do we do that? C is equals to two pi r remember we don't use this we use the pi second function pi times four and that will give us 25.13 and that's calculating the perimeter.