Question

the diameter of cone is 20 and its height is 28cm find its curved surface area

Answer 1

The curved surface area of a cone is given by the formula:
CSA (of cone)=πrl ( π=22/7:r=base radius of cone:l=slant height:h=height of cone )

since diameter is 20,( radius =diameter/2)=20/2=10 cm 

slant height l, is not given  hence to find the slant height use the formula
l²=h²+r²    (derived from pythagoras theorem)
l²=28²+10²
l²=784+100
l²=884
√l²=√884     (taking root on both sides)
l=29.73 ~ 30(approx)

Putting the value of l and r in the fomula of CSA we get
CSA(cone)=πrl
                 =22/7×10×30

                 =942 cm²

Answer 2

Hi there !!

We are given the diameter as 20 cm

So ,
Radius , r = 20 / 2 = 10 cm

Height (h) is also given as 28 cm.

Slant height = l

We know that

l² = h²+ r²

= 28 ² + 10²

= 784 + 100

= 884

l = √884

= 29.73 (approx.)


Curved Surface Area (CSA) = πrl

= 22 /7 × 10 × 29.73

= 934.37 cm²(approx.)