Question

Line Segment DE is parallel to side BC of right triangle ABC. CD = 3, DE = 6, and EB = 4. Compute the area of quadrilateral BCDE.

Answer 1

The area of quadrilateral BCDE = 20.4 sq. units

Let AD = x and AE = y.

Since ΔABC and ΔAED are similar right angled triangles, we have that

AC/AD = AB/AE

AC = AD + CD

= x + 3.

Also, AB = AE + EB

= y + 4

So, AC/AD = AB/AE

(x + 3)/x = (y + 4)/y

Cross-multiplying, we have

y(x + 3) = x(y + 4)

Expanding the brackets, we have

xy + 3y = xy + 4x

3y = 4x

y = 4x/3

In ΔAED, AD² + AE² = DE².

So, x² + y² = 6²

Substituting y = 4x/3 into the equation, we have

x² + y² = 6²

x² + (4x/3)² = 6²

x² + 16x²/9 = 36

(9x² + 16x²)/9 = 36

25x²/9 = 36

Multiplying both sides by 9/25, we have

x² = 36 × 9/25

Taking square root of both sides, we have

x = √(36 × 9/25)

x = 6 × 3/5

x = 18/5

x = 3.6

Since y = 4x/3,

Substituting x into the equation, we have

y = 4 × 3.6/3

y = 4.8

To find the area of quadrilateral BCDE, we subtract the area of ΔAED from area of ΔABC.

So, area of quadrilateral BCDE = area of ΔABC - area of ΔAED

area of ΔABC = 1/2 AC × AB

= 1/2 (x + 3)(y + 4)

= 1/2(3.6 + 3)(4.8 + 4)

= 1/2 × (6.6)(8.8)

= 1/2 × 58.08

= 29.04  square units

area of ΔAED = 1/2 AD × AE

= 1/2xy

= 1/2 × 3.6 × 4.8

= 1/2 × 17.28

= 8.64 square units

area of quadrilateral BCDE = area of ΔABC - area of ΔAED

area of quadrilateral BCDE = 29.04 sq units - 8.64 sq units

area of quadrilateral BCDE = 20.4 sq. units

So, the area of quadrilateral BCDE = 20.4 sq. units

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