Giving figure..

Red part is the considered element..

tan @ =(b - a ) /  h =(b - r) / x...        (REFER FIGURE)

Solve these two and you get r = b- (b - a)* x/h...substitute this value...

for tan@, substitute (b- a)/h...

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I think that should clear it...

if there's still some doubt, temme,i'll show d integration..

do rate if it helped!