what is sqrt( 1+(dy/dx)^2 ) and that multiplied by (dy/dx)
Is your first dy/dx term x^(3/8) or (x^3)/8 ? Is the second term (1/2)x^2 or 1/(2x^3) ? It seems to me all you have to do is substitute the dy/dx you already know into whatever you want to calculate.
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