Similar Triangles Calculator
Calculate similar triangles instantly with our math tool. Shows detailed work, formulas used, and multiple solution methods.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
Scale Factor (k) = Side2 / Side1 | Area Ratio = k^2 | Perimeter Ratio = k
Where k is the scale factor found by dividing a known side of the second triangle by its corresponding side in the first triangle. All corresponding sides share this same ratio. The area ratio equals k squared, while the perimeter ratio equals k.
Worked Examples
Example 1: Finding Unknown Sides with Scale Factor
Problem:Triangle ABC has sides a=3, b=4, c=5. Triangle DEF is similar with side d=6 (corresponding to a). Find the remaining sides of DEF.
Solution:Scale factor k = d/a = 6/3 = 2\ne = b x k = 4 x 2 = 8\nf = c x k = 5 x 2 = 10\nPerimeter of ABC = 3 + 4 + 5 = 12\nPerimeter of DEF = 6 + 8 + 10 = 24\nArea of ABC = 6 (using Heron formula)\nArea of DEF = 6 x 4 = 24 (area ratio = k squared = 4)
Result:Side e = 8, Side f = 10, Scale Factor = 2, Area Ratio = 4
Example 2: Height of a Building Using Shadows
Problem:A 2m stick casts a 1.5m shadow. A building casts a 30m shadow at the same time. Find the building height using similar triangles.
Solution:The stick and building form similar triangles with the sun rays.\nStick height / Stick shadow = Building height / Building shadow\n2 / 1.5 = Building height / 30\nBuilding height = (2 x 30) / 1.5 = 60 / 1.5 = 40m\nScale factor = 30 / 1.5 = 20
Result:Building height = 40 meters, Scale Factor = 20
Frequently Asked Questions
What are similar triangles and how are they defined?
Similar triangles are triangles that have exactly the same shape but may differ in size. Two triangles are similar when all three pairs of corresponding angles are equal, and all three pairs of corresponding sides are in the same proportion. This means one triangle is essentially a scaled version of the other. The symbol used for similarity is a tilde (~). Similarity is a fundamental concept in Euclidean geometry and appears frequently in proofs, architectural design, and surveying applications. Unlike congruent triangles which must be identical in size, similar triangles can be any size as long as the proportions and angles match perfectly.
What are the criteria for proving two triangles are similar?
There are three main criteria for proving triangle similarity. The AA (Angle-Angle) criterion states that if two angles of one triangle equal two angles of another, the triangles are similar, since the third angles must also be equal. The SAS (Side-Angle-Side) similarity criterion requires two pairs of proportional sides with the included angle being equal. The SSS (Side-Side-Side) similarity criterion requires all three pairs of corresponding sides to be in the same ratio. The AA criterion is the most commonly used because you only need to show two angle pairs are equal, which is often the easiest condition to verify in geometric proofs and practical applications.
How do you find the scale factor between similar triangles?
The scale factor between similar triangles is found by dividing any corresponding side of one triangle by the matching side of the other triangle. For example, if triangle ABC has sides 3, 4, 5 and triangle DEF has sides 6, 8, 10, the scale factor is 6/3 = 8/4 = 10/5 = 2. This ratio must be the same for all three pairs of corresponding sides. If the scale factor is greater than 1, the second triangle is larger (an enlargement). If it is less than 1, the second triangle is smaller (a reduction). The scale factor is crucial because it allows you to find any unknown side length by multiplying the known corresponding side by this ratio.
What are some real-world applications of similar triangles?
Similar triangles have numerous practical applications across many fields. In surveying and navigation, they are used to measure heights and distances that cannot be measured directly, such as determining the height of a building using its shadow and the shadow of a known object. Architects use similar triangles when creating scale models of buildings. In photography, similar triangles explain how lens focal length relates to image size and distance. Map-making relies heavily on similar triangles to maintain accurate proportions when reducing real-world distances to map scale. Engineers use them in structural analysis, and artists apply the principles of similar triangles in perspective drawing to create realistic depth.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy