A0S4 b=Sua+S2a2=SuE n-e-)î í ì = + = = + = b S a S a S E b S a S a S E 2 21 1 22 2 21 1 11 1 12 2 11 ( ) 2 2 21 2 2 2 2 1 1 2 1 P L = b - G L = S E