Question

Given: △DMN, DM=10 3 m∠M=75°, m∠N=45° Find: Perimeter of △DMN

Answer 1

Angle D is 180° -75° -45° = 60°. Drawing altitude MX to segment DN divides the triangle into ΔMDX, a 30°-60°-90° triangle, and ΔMNX, a 45°-45°-90° triangle. We know the side ratios of such triangles (shortest-to-longest) are ...

... 30-60-90: 1 : √3 : 2

... 45-45-90: 1 : 1 : √2

The long side of ΔMDX is 10√3, so the other two sides are

... MX = MD(√3/2) = 15

... DX = MD(1/2) = 5√3

The short side of ΔMNX is MX = 15, so the other two sides are

... NX = MX(1) = 15

... MN = MX(√2) = 15√2

Then the perimeter of ΔDMN is ...

... P = DM + MN + NX + XD

... P = 10√3 +15√2 + 15 + 5√3

... P = 15√3 +15√2 +15 . . . . perimeter of ΔDMN