Question

The diameter of circle is 21 cm find its circumference with solution

Answer 1

Hey !!! ^_^


Here is your answer

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The diameter of circle is 21 ..
Radius = 21/2

As we know that ,


The formula to find circumference = 2πr

$2 \times \frac{22}{7} \times \frac{21}{2} \\ \\ \frac{22}{7} \times 21 \\ \\ 22 \times 3 \\ \\ 66..cm$



The circumference of circle is 66 cm ..


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I HOPE IT WILL HELP YOU


Thank you

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Answer 2

Assigning the names

Okay from now on:

Circumference will be known as C.

diameter will be known as d.

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Pi(the constant) derivation

Okay we learnt in class 8 the circumference of a circle is equal to 2πr

Now how did this come.We should know about its origin to believe that this formula is correct.

      Now this is an experiment:

• Draw a circle of any radius.
• Measure the diameter.
• Measure its circumference with a string or something like that.
• You will notice that:-

             C∝d

Circumference varies directly with diameter.So if circumference is to be increased just draw a circle having big diameter or radius.

Now we remove the ∝ sign with uh: a constant.

This constant never changes its value whatever the diameter whatever the circumference.

So :    C=πd

We know d=2r obviously the diameter is twice the radius.

So :  C=2πr.

We hence got this formula.

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Application of formula(just what students blindly do)

Which one is more easy :Ok the first one :

C=πd.

Wait here it is given that diameter is 21cm.

Oh just forgot to tell you the value of pi.

Pi=3.14 (this is very complex)      

 Pi=$\frac{22}{7}$

Now this is easy.

C=πd.

Put the value of pi and diameter to get the solution.

$C=\frac{22}{7}*21$cm

Come on we have to multiply 21 with 22 just get lost this is boring.

Wait we can diving 21 and 7 to get 3.

$C=22*3$cm

$C=66$cm.

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Answer

We found the circumference to be 66cm.

66cm

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Hope this helps you.

Just enjoy playing with mathematics. It is the most interesting subject ever.

Thanks for the question.

:-)