Examples with solutions for Using the Pythagorean Theorem: In combination with the triangle area

Exercise #1

The area of the triangle ABC is 30 cm².

What is the length of the hypotenuse?

S=30S=30S=30555AAABBBCCC

Video Solution

Step-by-Step Solution

To find the length of the hypotenuse of the triangle, let's proceed with the following steps:

  • Step 1: Use the area formula to find the base.
  • Step 2: Apply the Pythagorean Theorem to find the hypotenuse.

Step 1:
The formula for the area of a right-angled triangle is:

Area=12×base×height\text{Area} = \frac{1}{2} \times \text{base} \times \text{height}

Given that the area is 30 cm², and one leg (the height) is 5 cm, we can find the base:

12×base×5=30\frac{1}{2} \times \text{base} \times 5 = 30

Solve for base\text{base}:

12×base×5=30\frac{1}{2} \times \text{base} \times 5 = 30

Multiply both sides by 2:

5×base=605 \times \text{base} = 60

Divide by 5:

base=12cm\text{base} = 12 \, \text{cm}

Step 2:
With base and height (legs of the triangle) known, apply the Pythagorean theorem to find the hypotenuse, c c :

c2=(base)2+(height)2=122+52c^2 = (\text{base})^2 + (\text{height})^2 = 12^2 + 5^2

Calculate:

c2=144+25=169c^2 = 144 + 25 = 169

Take the square root to find c c :

c=169=13cmc = \sqrt{169} = 13 \, \text{cm}

Therefore, the length of the hypotenuse of the triangle is 13 cm.

Answer

13 cm

Exercise #2

The area of triangle ABC is equal to 2X+16 cm².

Work out the value of X.

333X+5X+5X+5BBBAAACCCDDD

Video Solution

Step-by-Step Solution

The area of triangle ABC is equal to:

AD×BC2=2x+16 \frac{AD\times BC}{2}=2x+16

As we are given the area of the triangle, we can insert this data into BC in the formula:

AD×(BD+DC)2=2x+16 \frac{AD\times(BD+DC)}{2}=2x+16

AD×(x+5+3)2=2x+16 \frac{AD\times(x+5+3)}{2}=2x+16

AD×(x+8)2=2x+16 \frac{AD\times(x+8)}{2}=2x+16

We then multiply by 2 to eliminate the denominator:

AD×(x+8)=4x+32 AD\times(x+8)=4x+32

Divide by: (x+8) (x+8)

AD=4x+32(x+8) AD=\frac{4x+32}{(x+8)}

We rewrite the numerator of the fraction:

AD=4(x+8)(x+8) AD=\frac{4(x+8)}{(x+8)}

We simplify to X + 8 and obtain the following:

AD=4 AD=4

We now focus on triangle ADC and by use of the Pythagorean theorem we should find X:

AD2+DC2=AC2 AD^2+DC^2=AC^2

Inserting the existing data:

42+(x+5)2=(65)2 4^2+(x+5)^2=(\sqrt{65})^2

16+(x+5)2 =65/16 16+(x+5)^2\text{ }=65/-16

(x+5)2=49/ (x+5)^2=49/\sqrt{}

x+5=49 x+5=\sqrt{49}

x+5=7 x+5=7

x=75=2 x=7-5=2

Answer

2 cm

Exercise #3

The area of the triangle ABC is 4X+16 cm².

Express the length AD in terms of X.

S=4X+16S=4X+16S=4X+16X+4X+4X+4AAABBBCCCDDD

Video Solution

Step-by-Step Solution

The area of triangle ABC is:

AB×AC2=S \frac{AB\times AC}{2}=S

Into this formula, we insert the given data:

AB×(x+4)2=4x+16 \frac{AB\times(x+4)}{2}=4x+16

AB×(x+4)2=4(x+4) \frac{AB\times(x+4)}{2}=4(x+4)

Notice that X plus 4 on both sides is reduced, and we are left with the equation:

AB2=4 \frac{AB}{2}=4

We then multiply by 2 and obtain the following:

AB=4×2=8 AB=4\times2=8

If we now observe the triangle ABC we are able to find side BC using the Pythagorean Theorem:

AB2+AC2=BC2 AB^2+AC^2=BC^2

We first insert the existing data into the formula:

82+(x+4)2=BC2 8^2+(x+4)^2=BC^2

We extract the root:

BC=64+x2+2×4×x+42=x2+8x+64+8=x2+8x+72 BC=\sqrt{64+x^2+2\times4\times x+4^2}=\sqrt{x^2+8x+64+8}=\sqrt{x^2+8x+72}

We can now calculate AD by using the formula to calculate the area of triangle ABC:

SABC=AD×BC2 S_{\text{ABC}}=\frac{AD\times BC}{2}

We then insert the data:

4x+16=AD×x2+8x+802 4x+16=\frac{AD\times\sqrt{x^2+8x+80}}{2}

AD=(4x+16)×2x2+8x+80=8x+32x2+8x+80 AD=\frac{(4x+16)\times2}{\sqrt{x^2+8x+80}}=\frac{8x+32}{\sqrt{x^2+8x+80}}

Answer

8x+32x2+8x+80 \frac{8x+32}{\sqrt{x^2+8x+80}}

Exercise #4

Given the triangles in the figure

The triangles are similar and the ratio is 1:2

DEF the little one among them

Find the area of the triangle DEF

444AAABBBCCCDDDEEEFFF

Video Solution

Answer

4 cm²

Exercise #5

Express the area of the triangle ABC in terms of X.

2X2X2XAAABBBCCCDDD8X+1

Video Solution

Answer

X+923X22X1 \frac{X+9}{2}\sqrt{3X^2-2X-1}