Final answer:
Einthoven's triangle is formed by leads I, II, and III, which are placed on the limbs of the body to record the electrical activity of the heart.
Step-by-step explanation:
Einthoven's triangle is formed by leads I, II, and III. These leads are placed on the limbs of the body to record the electrical activity of the heart. Lead I is placed on the left arm, lead II is placed on the right arm, and lead III is placed on the left leg. Together, these leads create a triangle shape known as Einthoven's triangle.