8.解ï¼âµz=x^2+y^2
â´Î±z/αx=2xï¼Î±z/αy=2y
æ
ææ±é¢ç§¯=â«â«<S>dS (Sæ¯åå½¢åºåï¼x^2+y^2â¤9)
=â«â«<S>â[1+(αz/αx)^2+(αz/αy)^2]dxdy
=â«â«<S>â(1+4x^2+4y^2)dxdy
=â«<0,2Ï>dθâ«<0,3>â(1+4r^2)rdr (ä½æåæ åæ¢)
=(Ï/4)â«<0,3>â(1+4r^2)d(1+4r^2)
=(Ï/4)[(2/3)(37^(3/2)-1)]
=(Ï/6)(37â37-1)ã
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