1.2*3/2=32.(2+6)*4/2=163.(6+1)*3/2+4*(1+2)/2-(6+1)*4/2=5/24.(5+3)*4/2=16 设三点(x1,y1)(x2,y2)(x3,y3)S面积=|y1-y3|*|x2-x1|/2+|x3-x1|*|y2-y3|/2-|x1-x3|*|y1-y3|/2
