S 2 ∑x( ④计算机法 s=∑y(x X2,y2 S=(S12+S23)-(S51+ (x1+x2)(y2-y1 (x2+x3)(y3y2)2 y1)/2 X5Ly5 y5 S=x1(y2y5)+x2(y3y1)+ 3(ys-y4)+x41(ysy3)+④计算机法 x1 , y1 x5 , y5 x4 , y4 x2 , y2 x3 , y3 y1 y5 y2 y4 y3 x1 x2 x3 x4 x5 S = (s12+ s23)－(s51+ s45+ s34) = (x1+ x2 ) (y2 - y1 )/2 + (x2+ x3 ) (y3 - y2 )/2 － (x1+ x5 ) (y5 - y1 )/2 － (x4+ x5 ) (y4 - y5 )/2 － (x3+ x4 ) (y3 - y4 )/2 S =[x1 (y2 -y5 )+ x2 (y3 -y1 )+ x3 (y5 -y4 )+ x41 (y5 -y3 )+ x5 (y1 -y4 )]/2 ( ) 2 1 ( ) 2 1 1 1 1 1 1 1 − = + − = +   = − = − i n i i i i n i i i s y x x s x y y