What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Two triangles are Similar if the only difference is size (and possibly the need to turn or flip one around).

These triangles are all similar:

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

(Equal angles have been marked with the same number of arcs)

Some of them have different sizes and some of them have been turned or flipped.

For similar triangles:

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

All corresponding angles are equal

and

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

All corresponding sides have the same ratio

Also notice that the corresponding sides face the corresponding angles. For example the sides that face the angles with two arcs are corresponding.

Corresponding Sides

In similar triangles, corresponding sides are always in the same ratio.

For example:

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Triangles R and S are similar. The equal angles are marked with the same numbers of arcs.

What are the corresponding lengths?

  • The lengths 7 and a are corresponding (they face the angle marked with one arc)
  • The lengths 8 and 6.4 are corresponding (they face the angle marked with two arcs)
  • The lengths 6 and b are corresponding (they face the angle marked with three arcs)

Calculating the Lengths of Corresponding Sides

We can sometimes calculate lengths we don't know yet.

  • Step 1: Find the ratio of corresponding sides
  • Step 2: Use that ratio to find the unknown lengths

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Step 1: Find the ratio

We know all the sides in Triangle R, and
We know the side 6.4 in Triangle S

The 6.4 faces the angle marked with two arcs as does the side of length 8 in triangle R.

So we can match 6.4 with 8, and so the ratio of sides in triangle S to triangle R is:

6.4 to 8

Now we know that the lengths of sides in triangle S are all 6.4/8 times the lengths of sides in triangle R.

Step 2: Use the ratio

a faces the angle with one arc as does the side of length 7 in triangle R.

a = (6.4/8) × 7 = 5.6

b faces the angle with three arcs as does the side of length 6 in triangle R.

b = (6.4/8) × 6 = 4.8

Done!

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Did You Know?

Similar triangles can help you estimate distances.

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What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?
What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

In this lesson we’ll look at the ratios of similar triangles to find out missing information about similar triangle pairs.

Similar triangles

In a pair of similar triangles, corresponding sides are proportional and all three angles are congruent. This means if you know two triangles are similar to one another you can use the information to solve for missing parts.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Corresponding angles

In a pair of similar triangles the corresponding angles are the angles with the same measure. In the diagram of similar triangles, the corresponding angles are the same color.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Corresponding sides

In a pair of similar triangles, the corresponding sides are proportional. Corresponding sides touch the same two angle pairs.

When the sides are corresponding it means to go from one triangle to another you can multiply each side by the same number.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

In the diagram of similar triangles the corresponding sides are the same color.

Naming similar triangles

To show two triangles are similar, you use the symbol ???\sim???. You need to match the letters from the first triangle to the angles with the corresponding vertices on the second triangle.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Here we can say that ???\triangle XYZ\sim \triangle BCA???.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?
What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Example

If the two triangles in the diagram are similar, solve for the variable.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

In similar triangles, corresponding sides are proportional. In the diagram, ???x??? corresponds to ???8???, and ???42.5??? corresponds to ???17???. So we say

???\frac{x}{8}=\frac{42.5}{17}???

???17x=8(42.5)???

???17x=340???

???x=20???

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

Example

If ???\triangle XVY\sim \triangle XWZ???, solve for ???x???.

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

In similar triangles, corresponding sides are proportional. In the diagram, ???4x+12??? corresponds to ???88???, and ???91??? corresponds to ???143???.

???\frac{4x+12}{88}=\frac{91}{143}???

???(4x+12)143=91(88)???

???(4x+12)143=8,008???

???\frac{(4x+12)143}{143}=\frac{8,008}{143}???

???4x+12=56???

???4x=44???

???x=11???

What kind of triangle is it if the triangles are the same shape the corresponding angles of both triangles are congruent and the corresponding sides are proportional?

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