Q&A for How to Find the Perimeter of a Triangle

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  • Question
    Can you find the perimeter if only one side is given?
    wikiHow Staff Editor
    Staff Answer
    This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.
    wikiHow Staff Editor
    Staff Answer
    If you know only 1 side but all 3 angles, you can use the rule of sines to find the remaining sides, then calculate the perimeter. If you know 1 side and 1 angle, you won’t be able to find the perimeter unless you’re dealing with a right triangle. For right triangles, you know that 1 angle is always 90°, so if you know another angle, you can use the sum of angles (180°) to figure out the third one. From there, you can use the laws of sine and cosine to figure out the other sides.
  • Question
    How can you find the perimeter of a triangle if one side is missing?
    wikiHow Staff Editor
    Staff Answer
    This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.
    wikiHow Staff Editor
    Staff Answer
    If it’s a right triangle, you can use the Pythagorean theorem (a2 + b2 = c2) to find the length of the missing side. From there, you can easily calculate the perimeter. For other types of triangles, you can use the law of cosines to find the perimeter if you know 2 sides and at least 1 of the angles.
  • Question
    How can you find the missing side of a triangle given the perimeter?
    wikiHow Staff Editor
    Staff Answer
    This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.
    wikiHow Staff Editor
    Staff Answer
    Since the perimeter is the sum of the lengths of all sides, you can solve for the missing side by subtracting the lengths of the other 2 sides from the perimeter.
  • Question
    Can I solve the perimeter of a triangle if I'm only given three included angles?
    Orangejews
    Community Answer
    No. You can know the exact shape of the triangle from the angles but nothing about its size, so the perimeter could be anything. (At least for Euclidean geometry; in spherical geometry, all similar triangles are congruent so you do know the scale and thus the perimeter.)
  • Question
    Two sides of a triangle are each 7 cm long. The length of the third side is an integer number of centimeters. At most how many centimeters do the perimeter of the triangle measure?
    Orangejews
    Community Answer
    Triangle inequality says no side can be longer than the sum of the other two. If the third side were 14 cm, it would make a line segment, not a triangle, but the next integer smaller 13 cm works. That makes a perimeter of 27 cm.
  • Question
    The base of an isosceles triangle is 4/3 cm. The perimeter of the triangle is 42/15 cm. What is the length of either of the remaining equal sides?
    Top Answerer
    Subtract 4/3 from 42/15, then divide by 2.
  • Question
    How do I determine the perimeter of a triangle when I only know the base and height?
    Community Answer
    You can't find the perimeter with just the base and height because you need the measure of at least two sides and an angle between them.
  • Question
    If an angle is 5 degrees and two sides are 10 feet, what is the formula to figure out the third side?
    Top Answerer
    It depends on where the angle is situated in relation to the two sides.
  • Question
    How do I find a perimeter of where sides a = 8.1, b = 16.1, c = 18 and height = 6?
    Top Answerer
    If you know all three side lengths, that's all you need. Add the three sides together. The height is not needed.
  • Question
    One side of triangle is 13 cm and another side is 17 cm, so what can be the perimeter of triangle?
    Orangejews
    Community Answer
    The triangle inequality means the third side must be between 4 and 30cm, so the perimeter is between 34 and 60 cm.
  • Question
    How can I find the perimeter of a triangle when only given the vertices of the triangle on a coordinate plane?
    Top Answerer
    Calculate the length of each side using the Pythagorean theorem, and add them together.
  • Question
    The perimeter of a triangle is 74 cm, and the longest side is 6 cm less than the sum of the other two sides. Twice the shortest side is 4 cm less than the longest side. How do I find the length of each side?
    Top Answerer
    Set up three equations in three unknowns: a + b + c = 74; a = (b + c) - 6; and c = a - 4. Solve by substitution, such as substituting (a - 4) for c in the first two equations and solving those two simultaneously.
  • Question
    How do I find the perimeter of a triangle, given the length of two sides?
    Top Answerer
    You'd also need to know the angle between the sides.
  • Question
    How do I calculate the perimeter of a triangle when area is given?
    Top Answerer
    If all you know is the area, it's not possible to find the perimeter.
  • Question
    How do I work out the perimeter of a right-angle triangle with only one side known? The hypotenuse is 25, one angle is 45, another 90.
    Top Answerer
    This triangle is isosceles, because the third angle is also 45°. (180° - 90° - 45° = 45°.) The Pythagorean theorem (a² + b² = c²) tells us that the sum of the squares of the two legs of a right triangle equals the square of the hypotenuse. Since in this triangle a=b, a² + a² = 2a² = c². The hypotenuse is given as 25, so c²=625. Therefore, 2a² = 625. Then a²=312.5, and a=17.67. That's what b equals, too. So the perimeter is 25 + 17.67 + 17.67 = 60.34.
  • Question
    The length and width of a rectanle are in the ratio of 4:7. Find the area of the rectanle if its perimeter is 66 units.
    Top Answerer
    The lengths of the rectangle's four sides are in the ratio of 4:7:4:7. Add those four numbers together to get 22. 66÷22 = 3. Multiply 3 by the four-side ratio to get the actual side lengths of 12, 21, 12 and 21 units. The rectangle's area is (12)(21) = 252 square units.
  • Question
    How do I find the area of a triangle when I know the perimeter?
    Community Answer
    Perimeter of a triangle and its area are not correlated, they are entirely different quantities. You cannot find one from the other without some more information about the triangle.
  • Question
    Two sides of an isosceles triangle are 8 and 17. Compute its perimeter.
    Top Answerer
    It can't be done. The third side would be either 8 or 17, but we don't know which.
  • Question
    The perimeter is 38, length is wide +3, what is the width?
    Top Answerer
    Let's assume you're referring to a rectangle. Let w be the width and (w+3) be the length. Then w + w + (w+3) + (w+3) = 38. So 2w + 2(w+3) = 2w + (2w + 6) = 4w + 6 = 38. Then 4w = 32, and w = 8. So the width is 8.
  • Question
    How can I find the perimeter from the area of a triangle?
    Top Answerer
    If all you know is the area, you don't have enough information to find the perimeter. That's because with a given area, the base could be many different lengths.
  • Question
    How do I find the perimeter when given three points?
    Top Answerer
    See Find the Distance Between Two Points . Use that technique to find the distance between each pair of points. Then add the three distances together.
  • Question
    How do you find the perimeter of an equivalent triangle when the side length is x+3?
    Top Answerer
    If you're talking about an equilateral triangle, the perimeter is three times the side length, which means in this case 3(x + 3) = 3x + 9.
  • Question
    How can I learn trigonometry?
    Top Answerer
    See Learn Trigonometry and related articles.
  • Question
    Perimeter of a triangle when it's a right triangle. The angles are 45+45+90. One side is 4.9. How do we figure the length of the other two sides?
    Top Answerer
    You would first have to know whether the 4.9 side is the hypotenuse or one of the legs. The ratio of the sides of a triangle with those angles is 1:1:√2. So you would use that ratio and the 4.9 dimension to calculate the other two sides and then the perimeter.
  • Question
    How do I find the perimeter if the triangle is in ratio of 3:4:5 and I know the longest side?
    Community Answer
    Multiply it by 3/5 and 4/5 to find the other sides, then add all three together.
  • Question
    How do I find the perimeter of a triangle if I know the sides?
    Community Answer
    Simply add the lengths of the sides together to find the perimeter.
  • Question
    How do I find the sides of the triangle when only the perimeters are given?
    Community Answer
    If by "sides" you mean two or three sides of the given triangle, then it isn't possible to find them with only the perimeter, (unless it is an isosceles triangle). You need to know at least two sides or angles (depending on the number of sides you need to find) in order to use perimeter and find the unknown side(s). As for an isosceles triangle, since all the three sides are the same length, you can write the perimeter formula as; perimeter = 3*x (x being the sides) calculate x and there you have all three sides of the that triangle. If you want, you can double check your answer by checking if 3*x = perimeter, or not.
  • Question
    How do I find the perimeter of a triangle if I know the hypotenuse?
    Community Answer
    You can't. The formula for a triangle's perimeter is 3*sides, or more commonly written as A+B+C (they are the same thing). If all you know is the hypotenuse, you can't find the perimeter of the triangle; neither directly with the above formula, nor indirectly with the Pythagorean theorem. You need to know at least one more side or 2 angles.
  • Question
    How can I proof a triangle is a right angle triangle using the perimeter or Pythagoras theorem?
    Community Answer
    The Pythagorean theorem is itself a proof that the triangle in question is a right triangle. The square of a right triangle's longest side i.e the hypotenuse - denoted by "c" - is always equal to the sum of the squares of the other two sides. Using Pythagorean theorem, you can easily proof that the said triangle is right or not. c^2 (hypotenuse) = a^2 (base) + b^2 (perpendicular).
  • Question
    How do I work out three sides of a triangle when I know the perimeter is 52 meters, the longest side is 12 meters longer than the shortest and the third side is twice the length of the shortest side?
    Community Answer
    S + (S + 12) + (S x 2) = 52. So 4S + 12 = 52, 4S = 40, and S = 10. So the shortest side is 10 m, another side is 10 + 12 = 22 m, and the third side is 2S = 20 m. The perimeter is 10 + 22 + 20 = 52 m.
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