Question

Seth is using the figure shown below to prove Pythagorean Theorem using triangle similarity. In the given triangle ABC, angle A is 90° and segment AD is perpendicular to segment BC. The figure shows triangle ABC with right angle at A and segment AD. Point D is on side BC. Part A: Identify a pair of similar triangles. (2 points) Part B: Explain how you know the triangles from Part A are similar. (4 points) Part C: If DB = 9 and DC = 4, find the length of segment DA. Show your work. (4 points)

Answer 1

The relationship between the lengths of the sides of a right triangle are

given by Pythagoras theorem.

• Part A: ΔABC is similar to ΔADC

• Part B: ΔABC and ΔADC are similar according AA similarity postulate

• Part C: DA = 6

Reasons:

Part A:

∠A = 90°

Segment AD ⊥ Segment BC

Location of point D = Side BC

Part A: In triangle ΔABC, we have;

∠A = 90°, ∠B = 90° - ∠C

In triangle ΔADC, we have;

∠ADC = 90°, ∠DAC = 90° - ∠C

∴ ΔABC is similar to ΔADC by Angle-Angle, AA, Similarity Postulate

Part B: The triangles are similar according to AA similarity postulate,

because two angles in one triangle are equal to two angles in the other

triangle and therefore, by subtraction property of equality, the third angle

in both triangles are also equal.

Part C: The length of DB = 9

The length of DC = 4

Required: Length of segment DA

In triangle ΔABD, we have;

∠BDA = 90°= ∠ADC

∠DAC ≅ ∠B by Congruent Parts of Congruent Triangles are Congruent

Therefore;

ΔABD ~ ΔADC by AA similarity, which gives;

$\displaystyle \frac{\overline{DA}}{\overline{DC}} = \frac{\overline{BD}}{\overline{DA}}$

$\overline{DA}^2$ = $\overline{DC} \times \overline{BD}$

Which gives;

$\overline{DA}^2$ = 4 × 9 = 36

$\overline{DA}$ = √(36) = 6

$\overline{DA}$ = 6

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