Reaction is a response to action that is acting on the beam in the form of vertical forces.
In order to calculate reactions R1 and R2, one should must be familiar about taking moment and law of equillibrium.
Take moment about point D, for calculation of reaction R1.
\(\sum M_{D}\space\ = 0\)
Law of Equilibrium says;
Clock wise moments = Counter clock wise moments
R1 x 6 = 400 x 1.5 + 1000 x 3
6R1 = 600 + 3000
6R1 = 3600
R1 = 600 Kg
Right now, we are familiar about value of reaction at point A. Reaction at point B can be calculated simply by subtracting R1 from sum of vertical forces.
\(\sum F_{V}\space\ = 0\)R1 + R2 = 1000 + 400
R2 = 1400 – R1
R2 = 1400 – 600
R2 = 800 Kg