Construct a triangle ABC, whose perimeter is 12.5 cm and whose base angles are 50° and 80°.

Step1: construct a segment XY of length perimeter which is 12.5 cm

Step2: draw ray at an angle of 50° from point X and a ray at 80° from point Y

Step3: draw angle bisectors using protractor of ∠X and ∠Y and mark their intersection point as A

Now we have to draw perpendicular bisector of line AX and AY

Step4: take any distance approximately by observation in compass greater than half of XA. Keep the needle of the compass on point A and mark arcs above and below XA and keeping the same distance in compass keep the needle on point X and cut the arcs as shown and join both the intersected arcs. Thus we have drawn the perpendicular bisector of AX. Mark the intersection point of the perpendicular bisector of AX and XY as point B

Step5: similarly draw perpendicular bisector of segment AY and mark the intersection point with XY as C join AB and AC and required ΔABC is ready