3. Using ruler and compasses only, construct an angle of $90^{\circ}$ at point $\mathrm{D}$ on the line $\mathrm{DE}$ below. D E

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.