Find out the equivalent thermal resistance between point A and B.

Explanation

`dR=(dx)/(dpir^2)`
`r_1/y=(r_2)/(r+l)=(r )/(r+x)`
`rArr r_1 y+r_1 l=r_2 y`
`y=(-r_1 l)/((r_1-r_2))=(r_1 l)/((r_2-r_1))`
`rArr r=(r_2 (y+x))/(y+l)=(r_2((r_2 l)/(r_2-r_1)+x))/(((r_1l)/(r_2-r_1)+l))`

`dR=(dx)/(k pi r^2) `
`rArr underset(0) overset(l)(int) (dx)/(k.pir^2 )=underset(0) overset(l)(int) (dx)/(k.pi(r_1+((r_2-r_1)x)/(l))^2)" "...(i)`