I probably understand it.OK I see. This formula "Rth=0.1(1+β)Re" is a rule of thumb used sometimes by a designer. And this rule means that we must choose R1||R2 < 0.1(1+β)Re for "bias-stable". This rule ensure that our voltage divider voltage is "stiff". Because base current is a load for our voltage divider. And to ensure that our divider voltage do not change much after we loading it we use this "rule of thumb" to select R1 and R2. The slightly different rule of thumb is to choose voltage divider current Id = Vcc/(R1+R2) at least 10 times the base current. So we have:

R1 = (Vcc - Vb)/(11*Ib)

R2 = Vb/(10*Ib)

But how to get these two equations?

R1 = (Vcc - Vb)/(11*Ib)

R2 = Vb/(10*Ib)