Use pythagorean theorem to find isosceles triangle side lengths calculator

The Pythagorean theorem can be used to solve for any unknown side of a right triangle if the lengths of the other two sides are known. The Pythagorean theorem can be used to solve for any side of an isosceles triangle as well, even though it is not a right triangle. Isosceles triangles have two sides of equal length and two equivalent angles. By drawing a straight line down the center of an isosceles triangle, it can be divided into two congruent right triangles, and the Pythagorean theorem can easily be used to solve for the length of an unknown side.

Draw your triangle upright on a piece of paper so the odd side (the one that is not equal in length to the other two) is at the base of the triangle. For example, assume an isosceles triangle with two sides of equal but unknown length, one side measuring 8 inches and a height of 3 inches. In your drawing, the 8 inch side should be at the base of the triangle.

Draw a straight line down the middle of the triangle from the vertex to the base. This line must be perpendicular to the base and divide the triangle into two congruent right triangles -- for this example, each with a height of 3 inches and a base of 4 inches.

Write the values of the lengths of the known sides of the triangle next to the sides they match. These values may come from a specific math problem or from measurements for a certain project. Write "3 in." next to the line drawn in Step 2 and "4 in." on either side of this line at the base of the triangle.

Determine which side is of unknown length and use the Pythagorean theorem to solve for it using a calculator. The unknown side is the hypotenuse of each of the two triangles.

Label the hypotenuse "C" and either of the legs of the triangle "A" and the other one "B."

Substitute the values for A, B and C into the Pythagorean theorem, (A)^2 + (B)^2 = (C)^2. For one of the two triangles constructed in this example, A = 3, B = 4 and C is what we are solving. Therefore, (3)^2 + (4)^2 = (C)^2 = 9 + 16 = 25. The square root of 25 is 5, so C = 5. The isosceles triangle we started with has two sides measuring 5 inches each and one side measuring 8 inches.

Things You'll Need

• Ruler
• Calculator

Tips

• The equation for the Pythagorean theorem is the square of the triangle's base added to the square of the triangle's height is equal to the square of the triangle's hypotenuse -- [(A)^2 + (B)^2 = (C)^2].

  The hypotenuse is the line that connects the base and height of a right triangle.

  The legs of a right triangle are the two sides that form the right angle.

  Use half of the original length of the base of the triangle as the base value for the right triangle, as you divided the triangle into two equal halves.

Video transcript

- [Instructor] We're asked to find the value of x in the isosceles triangle shown below. So that is the base of this triangle. So pause this video and see if you can figure that out. Well the key realization to solve this is to realize that this altitude that they dropped, this is going to form a right angle here and a right angle here and notice, both of these triangles, because this whole thing is an isosceles triangle, we're going to have two angles that are the same. This angle, is the same as that angle. Because it's an isosceles triangle, this 90 degrees is the same as that 90 degrees. And so the third angle needs to be the same. So that is going to be the same as that right over there. And since you have two angles that are the same and you have a side between them that is the same this altitude of 12 is on both triangles, we know that both of these triangles are congruent. So they're both going to have 13 they're going to have one side that's 13, one side that is 12 and so this and this side are going to be the same. So this is going to be x over two and this is going to be x over two. And so now we can use that information and the fact and the Pythagorean Theorem to solve for x. Let's use the Pythagorean Theorem on this right triangle on the right hand side. We can say that x over two squared that's the base right over here this side right over here. We can write that x over two squared plus the other side plus 12 squared is going to be equal to our hypotenuse squared. Is going to be equal to 13 squared. This is just the Pythagorean Theorem now. And so we can simplify. This is going to be x. We'll give that the same color. This is going to be x squared over four. That's just x squared over two squared plus 144 144 is equal to 13 squared is 169. Now I can subtract 144 from both sides. I'm gonna try to solve for x. That's the whole goal here. So subtracting 144 from both sides and what do we get? On the left hand side, we have x squared over four is equal to 169 minus 144. Let's see, 69 minus 44 is 25. So this is going to be equal to 25. We can multiply both sides by four to isolate the x squared. And so we get x squared is equal to 25 times four is equal to 100. Now, if you're just looking this purely mathematically and say, x could be positive or negative 10. But since we're dealing with distances, we know that we want the positive value of it. So x is equal to the principle root of 100 which is equal to positive 10. So there you have it. We have solved for x. This distance right here, the whole thing, the whole thing is going to be equal to 10. Half of that is going to be five. So if we just looked at this length over here. I'm doing that in the same column, let me see. So this length right over here, that's going to be five and indeed, five squared plus 12 squared, that's 25 plus 144 is 169, 13 squared. So the key of realization here is isosceles triangle, the altitudes splits it into two congruent right triangles and so it also splits this base into two. So this is x over two and this is x over two. And we use that information and the Pythagorean Theorem to solve for x.

Is it possible to apply the Pythagorean Theorem to an isosceles triangle?