The radius and height of a right circular cone are in the ratio of 5:12 and its volume is 107800 cm³.

(i) Find the radius of the cone.
(ii) Find the slant height of the cone.
(iii) Find the height of the cone.
(iv) Find the curved surface area of the cone.
(v) Find the total surface area of the cone.



Answer :

Let's tackle this problem step-by-step.

### Given:
- Volume of the cone [tex]\( V = 107800 \, \text{cm}^3 \)[/tex]
- The ratio of the radius to the height of the cone is [tex]\( 5:12 \)[/tex]

### To Find:
1. Radius of the cone [tex]\( r \)[/tex]
2. Slant height of the cone [tex]\( l \)[/tex]
3. Height of the cone [tex]\( h \)[/tex]
4. Curved surface area of the cone
5. Total surface area of the cone

### Step-by-Step Solution:

#### Let the radius be [tex]\( r = 5x \)[/tex] and the height be [tex]\( h = 12x \)[/tex].

#### 1. Find the radius of the cone [tex]\( r \)[/tex]
We know that:

[tex]\[ V = \frac{1}{3} \pi r^2 h \][/tex]

Substitute [tex]\( r = 5x \)[/tex] and [tex]\( h = 12x \)[/tex]:

[tex]\[ 107800 = \frac{1}{3} \pi (5x)^2 (12x) \][/tex]

Simplify:

[tex]\[ 107800 = \frac{1}{3} \pi 25x^2 12x \][/tex]
[tex]\[ 107800 = 100 \pi x^3 \][/tex]

Solve for [tex]\( x \)[/tex]:

[tex]\[ x^3 = \frac{107800}{100 \pi} \][/tex]
[tex]\[ x = \left(\frac{107800}{100 \pi}\right)^{1/3} \][/tex]

Using the appropriate calculations, we find:

[tex]\[ x \approx 7.00093902326458 \][/tex]

Now, the radius [tex]\( r \)[/tex] is:

[tex]\[ r = 5x \approx 5 \times 7.00093902326458 \approx 35.00469519685729 \, \text{cm} \][/tex]

#### 2. Find the height of the cone [tex]\( h \)[/tex]

[tex]\[ h = 12x \approx 12 \times 7.00093902326458 \approx 84.0112684724575 \, \text{cm} \][/tex]

#### 3. Find the slant height of the cone [tex]\( l \)[/tex]

Using the Pythagorean Theorem for the right triangle formed by the radius, height, and slant height:

[tex]\[ l = \sqrt{r^2 + h^2} \][/tex]

[tex]\[ l = \sqrt{(35.00469519685729)^2 + (84.0112684724575)^2} \][/tex]

[tex]\[ l \approx \sqrt{1225.3281988594 + 7057.891576019842} \][/tex]

[tex]\[ l \approx 91.01220751182896 \, \text{cm} \][/tex]

#### 4. Find the curved surface area of the cone

The formula for the curved surface area [tex]\( A_{\text{curved}} \)[/tex] is:

[tex]\[ A_{\text{curved}} = \pi r l \][/tex]

Substitute the known values:

[tex]\[ A_{\text{curved}} \approx \pi \times 35.00469519685729 \times 91.01220751182896 \][/tex]

[tex]\[ A_{\text{curved}} \approx 10008.657353812756 \, \text{cm}^2 \][/tex]

#### 5. Find the total surface area of the cone

The total surface area [tex]\( A_{\text{total}} \)[/tex] is the sum of the base area and the curved surface area:

[tex]\[ A_{\text{total}} = \pi r^2 + \pi r l \][/tex]

First, calculate the base area:

[tex]\[ \text{Base area} = \pi r^2 \approx \pi \times (35.00469519685729)^2 \][/tex]

[tex]\[ \text{Base area} \approx \pi \times 1225.3281988594 \approx 3849.48359762029 \, \text{cm}^2 \][/tex]

Adding the curved surface area:

[tex]\[ A_{\text{total}} \approx 3849.48359762029 + 10008.657353812756 \][/tex]

[tex]\[ A_{\text{total}} \approx 13858.140951433046 \, \text{cm}^2 \][/tex]

### Summary of Results:
1. Radius [tex]\( r \approx 35.00469519685729 \, \text{cm} \)[/tex]
2. Slant height [tex]\( l \approx 91.01220751182896 \, \text{cm} \)[/tex]
3. Height [tex]\( h \approx 84.0112684724575 \, \text{cm} \)[/tex]
4. Curved surface area [tex]\( \approx 10008.657353812756 \, \text{cm}^2 \)[/tex]
5. Total surface area [tex]\( \approx 13858.140951433046 \, \text{cm}^2 \)[/tex]