The two triangles below are similar. find the missing lengths

Kimoya W.

asked • 07/30/21

x= y=

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1 Expert Answer

Mahmut S. answered • 07/30/21

SAT Math Expert

Hello Kimoya.

According to similarity rules; there is a ratio between height of triangles; 12/4 = 3. that means corresponding sides of left triangle multiplied by 3 to get the sides of troangle on the right.

So 18/3 gives us x which is 6;

8*3 = y = 24

hope that helps

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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:

(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:


All corresponding angles are equal

and


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:

The two triangles below are similar. find the missing lengths

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

Example: Find lengths a and b of Triangle S

The two triangles below are similar. find the missing lengths

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!

Two triangles are similar if they have:

  • all their angles equal
  • corresponding sides are in the same ratio

But we don't need to know all three sides and all three angles ...two or three out of the six is usually enough.

There are three ways to find if two triangles are similar: AA, SAS and SSS:

AA

AA stands for "angle, angle" and means that the triangles have two of their angles equal.

If two triangles have two of their angles equal, the triangles are similar.

Example: these two triangles are similar:

The two triangles below are similar. find the missing lengths

If two of their angles are equal, then the third angle must also be equal, because angles of a triangle always add to make 180°.

In this case the missing angle is 180° − (72° + 35°) = 73°

So AA could also be called AAA (because when two angles are equal, all three angles must be equal).

SAS

SAS stands for "side, angle, side" and means that we have two triangles where:

  • the ratio between two sides is the same as the ratio between another two sides
  • and we we also know the included angles are equal.

If two triangles have two pairs of sides in the same ratio and the included angles are also equal, then the triangles are similar.

Example:

The two triangles below are similar. find the missing lengths

In this example we can see that:

  • one pair of sides is in the ratio of 21 : 14 = 3 : 2
  • another pair of sides is in the ratio of 15 : 10 = 3 : 2
  • there is a matching angle of 75° in between them

So there is enough information to tell us that the two triangles are similar.

Using Trigonometry

We could also use Trigonometry to calculate the other two sides using the Law of Cosines:

Example Continued

In Triangle ABC:

  • a2 = b2 + c2 - 2bc cos A
  • a2 = 212 + 152 - 2 × 21 × 15 × Cos75°
  • a2 = 441 + 225 - 630 × 0.2588...
  • a2 = 666 - 163.055...
  • a2 = 502.944...
  • So a = √502.94 = 22.426...

In Triangle XYZ:

  • x2 = y2 + z2 - 2yz cos X
  • x2 = 142 + 102 - 2 × 14 × 10 × Cos75°
  • x2 = 196 + 100 - 280 × 0.2588...
  • x2 = 296 - 72.469...
  • x2 = 223.530...
  • So x = √223.530... = 14.950...

Now let us check the ratio of those two sides:

a : x = 22.426... : 14.950... = 3 : 2

the same ratio as before!

Note: we can also use the Law of Sines to show that the other two angles are equal.

SSS

SSS stands for "side, side, side" and means that we have two triangles with all three pairs of corresponding sides in the same ratio.

If two triangles have three pairs of sides in the same ratio, then the triangles are similar.

Example:

The two triangles below are similar. find the missing lengths

In this example, the ratios of sides are:

  • a : x = 6 : 7.5 = 12 : 15 = 4 : 5
  • b : y = 8 : 10 = 4 : 5
  • c : z = 4 : 5

These ratios are all equal, so the two triangles are similar.

Using Trigonometry

Using Trigonometry we can show that the two triangles have equal angles by using the Law of Cosines in each triangle:

In Triangle ABC:

  • cos A = (b2 + c2 - a2)/2bc
  • cos A = (82 + 42 - 62)/(2× 8 × 4)
  • cos A = (64 + 16 - 36)/64
  • cos A = 44/64
  • cos A = 0.6875
  • So Angle A = 46.6°

In Triangle XYZ:

  • cos X = (y2 + z2 - x2)/2yz
  • cos X = (102 + 52 - 7.52)/(2× 10 × 5)
  • cos X = (100 + 25 - 56.25)/100
  • cos X = 68.75/100
  • cos X = 0.6875
  • So Angle X = 46.6°

So angles A and X are equal!

Similarly we can show that angles B and Y are equal, and angles C and Z are equal.

How do you find the length of a triangle with similar triangles?

Calculating the Lengths of Corresponding Sides.
Step 1: Find the ratio. We know all the sides in Triangle R, and. We know the side 6.4 in Triangle S. ... .
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..