### Video Transcript

In a square π΄π΅πΆπ·, π is the point of intersection of the two diagonals, πΈ is the midpoint of π΄π΅, and πΉ is the midpoint of π΅πΆ. Three forces of magnitudes πΉ one, πΉ two, and 41 newtons are acting at π in the directions ππΈ, ππΉ, and ππ·, respectively. Given that the three forces are in equilibrium, find the values of πΉ one and πΉ two.

Before starting this question, it is worth drawing a diagram. We have a square π΄π΅πΆπ· as shown. Weβre told that π is the point of intersection of the two diagonals, πΈ is the midpoint of π΄π΅, and πΉ is the midpoint of π΅πΆ. We also know that there are three forces acting at π. πΉ one acts in the direction ππΈ. πΉ two acts in the direction ππΉ. And finally, we have a 41-newton force acting in the direction ππ·. As the three forces are in equilibrium, there are several ways of solving this problem. In this question, we will use Lamiβs theorem.

This states that if three forces in equilibrium are acting at a point, in this case π΄, π΅, and πΆ, where the angle between forces π΅ and πΆ is πΌ, between π΄ and πΆ is π½, and between π΄ and π΅ is πΎ. Then π΄ over sin. πΌ is equal to π΅ over sin π½, which is equal to πΆ over sin πΎ. You may also notice that this is the sine rule formula from trigonometry. In our question, the angle between the force πΉ one and πΉ two is 90 degrees as they are perpendicular. The two diagonals of a square also meet at right angles. As half of a right angle is 45 degrees, the angle between πΉ one and the 41-newton force is 135 degrees. The angle between πΉ two and the 41-newton force is also 135 degrees as angles in a circle or at a point sum to 360.

Substituting these values into Lamiβs Theorem gives us 41 over sin 90 is equal to πΉ one over sin 135 which is equal to πΉ two over sin of 135. Letβs consider the first two terms. The sin of 90 degrees is equal to one. This means that 41 is equal to πΉ one over or divided by sin of 135. Multiplying both sides of this equation by sin of 135 gives us πΉ one is equal to 41 multiplied by sin 135. We could just type this into our calculator. However, we know that sin of 45 degrees is equal to one over root two. This can also be written as root two over two. If the sum of two angles equals 180 degrees, then the sin of both of these angles are equal. Therefore, the sin of 135 is also equal to root two over two.

This means that πΉ one is equal to 41 multiplied by root two over two. We can, therefore, say that the force πΉ one is equal to 41 root two over two newtons. If we consider the second and third term in Lamiβs Theorem, we noticed that the denominators are the same. Theyβre both sin of 135 degrees. If two fractions are equal and their denominators are equal, this means that the numerators must also be equal. As πΉ one is equal to 41 root two over two, then πΉ two must also be equal to 41 root two over two. Both of the forces in this case are 41 root two over two newtons.