3. Using ruler and compasses only, construct an angle of $90^{\circ}$ at point $\mathrm{D}$ on the line $\mathrm{DE}$ below.
D
E
Draw a line from D through G. This line will be perpendicular to DE, forming a 90 degree angle at D.
Step 1 :Open your compass to any radius r, and draw an arc cutting DE at D and some other point F.
Step 2 :Without changing the radius, place the compass at F and draw another arc cutting the previous arc at G.
Step 3 :Draw a line from D through G. This line will be perpendicular to DE, forming a 90 degree angle at D.