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Multiplication and Factorization of Mathematical Algebra Expressions in Grade Eight Chapter 14 decomposes factorial types with square difference formula.
The radius of the small circle is wrong.

Let the radius of the big circle be r and the radius of the small circle be r.

R+r=a......( 1)

R-r=b.......(2)

( 1)+(2):2R=a+b

R=(a+b)/2

( 1)-(2):2r=a-b

r=(a-b)/2

S yin =πR? -πr?

=π[(a+b)? /4-(a-b)? /4]

=π/4[(a+b+(a-b)][a+b-(a-b)]

=π/4(2a*2b)

=πab