Question 981752
The radius and height of a rught circular cone are 21 n 28cm. From its base to a height of 14 cm the portion of the curved surface was painted golden and the upper portiob as silver. Find the total cost of painting the whole curved surface at the rate of Rs50 /cm^2 for the golden colour and at the rate of Rs 40/cm^2 fir the silver colour. 

The radius circular cone is 21  
and height of a  28cm.

 From its base to a height of 14 cm 
The lower part is a frustum of a cone

*[illustration cone_painting].



Triangle AME and ANC are similar ( A A test of similarity)

14/28 = r1/21

r1= 10.5
By Pythagoras theorem AC^2 = AN^2 +NC^2
=28^2 +21^2
=1225
AC=35

Again 

AM/AN = AE/AC
1/2 = AE/AC 
1/2 = AE/35

AE = 17.5

The bottom portion is a frustum of cone
Curved surface area of frustum = pi*(r1+r2)*l=pi*(21+10.5)*17.5
=1732.5 cm^2
Multiply by cost Rs. 50/cm^2
= Rs. 86625

the curved surface of upper portion= pi*r*l

pi*10.5 * 17.5
= 577.26 cm^2

multiply by 40
=Rs 23,090.70

Add up both costs