Let the triangle be ABC 
∴ sides will be AB, BC , AC
Given : AB+BC=12cm and AB:BC=1:2 
 i.e, AB/BC=1/2
  ∴ 2AB=BC
Substituting the value of BC in 1st eqxn.. we get
2AB+AB=12 ⇒ 3AB=12 ⇒ ∴AB=4cm
∴BC=2AB=2(4)=8 cm
Now that you have got two sides and angle b/w them, you can construct the triangle ABC
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