DERIVATION OF AREA AND CIRCUMFERENCE OF CIRCLE USING CALCULUS
CIRCLE
DERIVATION OF AREA OF CIRCLE
LETS TAKE d๐ก , A VERY SMALL ANGLE IN ABOVE FIGURE
SO; THE SMALL PART OF CIRCLE IN DIAGRAM
CAN BE CONSIDERED AS RIGHT ANGLED TRIANGLE
tan d๐ก = ds/r
AS ANGLE IS VERY SMALL, tan d๐ก = d๐ก
SO, d๐ก =ds/r
SO, ds=r d๐ก ..................(1)
AREA OF TRIANGLE= 1/2 * BASE *HEIGHT
= 1/2 * r * ds
D(A) = 1/2 * r *r d๐ก............. FROM (1)
IF WE INTEGRATE THESE SMALL TRIANGLES HAVING THIS AREA, WE WILL GET
AREA OF CIRCLE
SO AREA OF CIRCLE= ∫D(A)
2๐
= ∫ 1/2* r * r d๐ก. [AS d๐ก IS FROM 0 TO 2๐ ]
0
[AS 2๐ REPRESENTS ANGLE OF ENTIRE CIRCLE]
2๐ 2
= ∫ (1/2) r d๐ก.
0
2 2๐
=(1/2 )* r * ∫ d๐ก
0
2
= [ r / 2 ] * 2๐
2
= ๐ r
SO, AREA OF CIRCLE IS ๐ซR*R
CIRCUMFERENCE OF CIRCLE
AS WE KNOW, ds=r d๐ก ............from 1
SO; CIRCUMFERENCE OF CIRCLE CAN BE OBTAINED BY ADDING ALL THESE LENGTHS
I.E INTEGRATING ds
SO C= ∫ ds
2๐ฅ
C = ∫ r d๐ก
0
2๐ฅ
C = r ∫ d๐ก
0
C= r * 2๐ฅ
C= 2๐ฅr
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