∑Fx=0 NB+NAx-qL/2=0
∑Fy=0 NAy-P=0
∑MA=0 (-qL/2)(2L/3)-P(b/2)+NB.L=0
联立解得：NB=qL/3+Pb/(2L)， NAy=P ，NAx =qL/6-Pb/(2L)
