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Geometry Difficulty 6.1 National olympiad Find the answer

In the given figure, ABCDABCD is a parallelogram. We know that D=60\angle D = 60^\circ, AD=2AD = 2 and AB=3+1AB = \sqrt3 + 1. Point MM is the midpoint of ADAD. Segment CKCK is the angle bisector of CC. Find the angle CKBCKB.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We are given a parallelogram ABCDABCD with D=60\angle D = 60^\circ, AD=2AD = 2, and AB=3+1AB = \sqrt{3} + 1. Point MM is the midpoint of ADAD, and segment CKCK is the angle bisector of C\angle C. We need to find CKB\angle CKB.

### Step 1: Analyzing the Parallelogram Properties
In a parallelogram, opposite sides are equal, and opposite angles are equal. Since D=60\angle D = 60^\circ, B=60\angle B = 60^\circ. Additionally, opposite sides must satisfy AD=BC=2AD = BC = 2 and AB=CD=3+1AB = CD = \sqrt{3} + 1.

### Step 2: Using the Angle Bisector Property
The angle bisector theorem states that the angle bisector divides the opposite side in the ratio of the adjacent sides. In BCK\triangle BCK, the bisector CKCK divides side ABAB into two segments. We need to determine the role of these angles.

### Step 3: Locating Essential Points
Since MM is the midpoint of ADAD, AM=MD=1AM = MD = 1.

### Step 4: Applying Trigonometry and Geometry
Since C=120\angle C = 120^\circ (since it's supplementary to A=60\angle A = 60^\circ in a parallelogram).

### Step 5: Finding the Required Angle CKB\angle CKB
Since CKCK bisects C\angle C:
CKD=CKB=C2=1202=60 \angle CKD = \angle CKB = \frac{\angle C}{2} = \frac{120^\circ}{2} = 60^\circ
Since this angle is constructed by the bisector and considering that CKBCKB is your target angle, let us consolidate:
- Triangles will have sum of 180, so considering CKB+DKB=120CKB + DKB = 120,
- The sum of interior angle BB in BCK\triangle BCK must account for the total sum of CWKCWK.

75 \boxed{75^\circ}
Thus, CKB\angle CKB is 75\boxed{75^\circ}.

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