How is it deduced to solve an equation in college physics electromagnetism? How are the equations such as question marks in photos derived?

The current I is evenly distributed on the cylindrical wall, and the circumference is divided into many dL. The current per dL di = IDL/2πr r.

The magnetic induction intensity of this current element dI at the center of the circle is DB = μ 0di/2π r, and the substitution of dI=IdL/2πR is dB= μ0IdL/2πR?

And dL= Rdθ is: db = μ 0 id θ/2 π r.

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