The vertex angle of an isosceles triangle is 90-50 = 40 or 180-(90-50) = 140.

The base angle = (180-40) ÷ 2 = 70, or (180- 140) ÷ 2 = 20.

tangram

The ratio of the area of each board to the area of a square is different, so the probability is not 1/7, depending on where the ants climb.

Q=600+50t

Look at the picture.

Pass through point p and make BC intersect AB in G, AC in H, ∫PD parallel AB, PE parallel AC,

Then quadrilaterals GBDP and HCEP are parallelograms. And BP and CP are bisectors of ∠ABC and ∠ACB respectively.

Then both GBDP and HCEP are diamonds.

BD=DP，EC=PE，

The circumference of △PDE is 5 cm.

Or (180- 140) ÷ 2 = 20.