ABCD is a cyclic quadrilateral. AB and DC meet at F, when produced. AD and BC meet at E, when produced. If ∠BAD = 68° and ∠AEB = 27°, then what is the measure of ∠BFC?
17°
Solving Cyclic Quadrilateral Angle Problems
This problem involves finding the measure of an angle formed by the intersection of produced sides of a cyclic quadrilateral. We are given the measure of one interior angle of the cyclic quadrilateral and the angle formed by the intersection of the other pair of produced sides.
Key Geometric Concepts
To solve this problem, we need to use the properties of cyclic quadrilaterals and the properties of angles in a triangle and angles on a straight line:
- Cyclic Quadrilateral Property: The sum of opposite angles in a cyclic quadrilateral is $180^\circ$.
- Angles in a Triangle: The sum of angles in any triangle is $180^\circ$.
- Linear Pair: Angles that form a straight line are supplementary; their sum is $180^\circ$.
Step-by-Step Solution for Finding ∠BFC
We are given that ABCD is a cyclic quadrilateral, ∠BAD = $68^\circ$, and ∠AEB = $27^\circ$, where E is the intersection of AD and BC produced, and F is the intersection of AB and DC produced.
Step 1: Find ∠BCD using Cyclic Quadrilateral Property
In a cyclic quadrilateral, opposite angles are supplementary. Therefore, ∠BAD + ∠BCD = $180^\circ$.
Given ∠BAD = $68^\circ$, we have:
\(\angle BCD = 180^\circ - \angle BAD\)
\(\angle BCD = 180^\circ - 68^\circ\)
\(\angle BCD = 112^\circ\)
Step 2: Analyse Triangle CDE
The lines AD and BC meet at E when produced. This means E lies on the extensions of both AD and BC. Let's assume AD is extended beyond D to E, and BC is extended beyond C to E. Thus, we have the line segments A-D-E and B-C-E forming triangle CDE.
In $\triangle CDE$, we have the angle at E, ∠AEB, which is $27^\circ$. So, ∠CED = ∠AEB = $27^\circ$.
The angle ∠DCE is formed by the line segment CE (part of the line BCE) and the line segment DC. Since B-C-E is a straight line, ∠BCE is a straight angle. ∠BCE = $180^\circ$. No, this is incorrect. ∠BCE is part of a straight line formed by extending BC to E. The angle ∠BCD is an interior angle of the cyclic quadrilateral. The angle ∠DCE and ∠BCD form a linear pair along the line BCE.
Thus, ∠DCE = $180^\circ - \angle BCD$ (Linear pair).
\(\angle DCE = 180^\circ - 112^\circ\)
\(\angle DCE = 68^\circ\)
The angle ∠CDE is formed by the line segment DE (part of the line ADE) and the line segment CD. Since A-D-E is a straight line, ∠ADE is a straight angle. The angle ∠ADC is an interior angle of the cyclic quadrilateral. The angle ∠CDE and ∠ADC form a linear pair along the line ADE.
Thus, ∠CDE = $180^\circ - \angle ADC$ (Linear pair).
The sum of angles in $\triangle CDE$ is $180^\circ$: ∠CED + ∠DCE + ∠CDE = $180^\circ$.
\(27^\circ + 68^\circ + (180^\circ - \angle ADC) = 180^\circ\)
\(95^\circ + 180^\circ - \angle ADC = 180^\circ\)
\(\angle ADC = 95^\circ\)
Step 3: Find ∠ABC using Cyclic Quadrilateral Property
In cyclic quadrilateral ABCD, opposite angles are supplementary. ∠ABC + ∠ADC = $180^\circ$.
\(\angle ABC = 180^\circ - \angle ADC\)
\(\angle ABC = 180^\circ - 95^\circ\)
\(\angle ABC = 85^\circ\)
Step 4: Analyse Triangle BCF
The lines AB and DC meet at F when produced. This means F lies on the extensions of both AB and DC. Let's assume AB is extended beyond B to F, and DC is extended beyond C to F. Thus, we have the line segments A-B-F and D-C-F forming triangle BCF.
In $\triangle BCF$, we have the angle at F, which is ∠BFC (what we need to find).
The angle ∠FBC is formed by the line segment BF (part of the line ABF) and the line segment BC. Since A-B-F is a straight line, ∠ABF is a straight angle. The angle ∠ABC is an interior angle of the cyclic quadrilateral. The angle ∠FBC and ∠ABC form a linear pair along the line ABF.
Thus, ∠FBC = $180^\circ - \angle ABC$ (Linear pair).
\(\angle FBC = 180^\circ - 85^\circ\)
\(\angle FBC = 95^\circ\)
The angle ∠FCB is formed by the line segment CF (part of the line DCF) and the line segment BC. Since D-C-F is a straight line, ∠DCF is a straight angle. The angle ∠BCD is an interior angle of the cyclic quadrilateral. The angle ∠FCB and ∠BCD form a linear pair along the line DCF.
Thus, ∠FCB = $180^\circ - \angle BCD$ (Linear pair).
\(\angle FCB = 180^\circ - 112^\circ\)
\(\angle FCB = 68^\circ\)
Step 5: Find ∠BFC using Angles in Triangle BCF
The sum of angles in $\triangle BCF$ is $180^\circ$.
\(\angle BFC + \angle FBC + \angle FCB = 180^\circ\)
\(\angle BFC + 95^\circ + 68^\circ = 180^\circ\)
\(\angle BFC + 163^\circ = 180^\circ\)
\(\angle BFC = 180^\circ - 163^\circ\)
\(\angle BFC = 17^\circ\)
Therefore, the measure of ∠BFC is $17^\circ$.
Summary of Calculations
| Angle | Calculation | Value | Reason |
|---|---|---|---|
| ∠BCD | $180^\circ - \angle BAD$ | $180^\circ - 68^\circ = 112^\circ$ | Opposite angles of cyclic quadrilateral |
| ∠DCE | $180^\circ - \angle BCD$ | $180^\circ - 112^\circ = 68^\circ$ | Linear pair (B-C-E is a line) |
| ∠CDE | $180^\circ - \angle CED - \angle DCE$ | $180^\circ - 27^\circ - 68^\circ = 85^\circ$ | Sum of angles in $\triangle CDE$ |
| ∠ADC | $180^\circ - \angle CDE$ | $180^\circ - 85^\circ = 95^\circ$ | Linear pair (A-D-E is a line) |
| ∠ABC | $180^\circ - \angle ADC$ | $180^\circ - 95^\circ = 85^\circ$ | Opposite angles of cyclic quadrilateral |
| ∠FBC | $180^\circ - \angle ABC$ | $180^\circ - 85^\circ = 95^\circ$ | Linear pair (A-B-F is a line) |
| ∠FCB | $180^\circ - \angle BCD$ | $180^\circ - 112^\circ = 68^\circ$ | Linear pair (D-C-F is a line) |
| ∠BFC | $180^\circ - (\angle FBC + \angle FCB)$ | $180^\circ - (95^\circ + 68^\circ) = 17^\circ$ | Sum of angles in $\triangle BCF$ |
Revision Table: Key Cyclic Quadrilateral Properties
| Property | Description |
|---|---|
| Opposite Angles | The sum of opposite interior angles is $180^\circ$. ∠A + ∠C = $180^\circ$, ∠B + ∠D = $180^\circ$. |
| Exterior Angle | An exterior angle is equal to the interior opposite angle. (If a side is produced) |
| Angles Subtended by Arc | Angles subtended by the same arc in the same segment are equal. |
| Angle in a Semicircle | The angle in a semicircle is a right angle ($90^\circ$). |
Additional Information: Angles from Intersecting Secants
When two secants are drawn to a circle from an external point, the angle formed at the external point is half the absolute difference of the measures of the intercepted arcs.
For the point E, formed by secants EAD and EBC, the intercepted arcs are arc AB and arc CD. The formula for $\angle E$ is:
\(\angle E = \frac{1}{2} | \text{measure of arc CD} - \text{measure of arc AB} |\)
For the point F, formed by secants FAB and FDC, the intercepted arcs are arc AD and arc BC. The formula for $\angle F$ is:
\(\angle F = \frac{1}{2} | \text{measure of arc BC} - \text{measure of arc AD} |\)
While these formulas provide an alternative method using arc measures, the approach using triangle angle sums and linear pairs is often more direct when given angles of the cyclic quadrilateral.
Important Questions from Quadrilaterals
What is the value of AC 2– BD 2
What is the area of the parallelogram?
ABCD is a cyclic quadrilateral. Diagonals BD and AC intersect each other at E. If ∠BEC = 138° and ∠ECD = 35°, then what is the measure of ∠BAC?
A circle is inscribed in a quadrilateral ABCD, touching sides AB, BC CD and DA at P, Q, R and S, respectively. If AS = 6 cm, BC = 12 cm, and CR = 5 cm, then the length of AB (in cm) is:
Sides AB and DC of a cyclic quadrilateral ABCD are produced to meet at E and sides AD and BC are produced to meet at F. If ∠ADC = 78° and ∠BEC = 52°, then the measure of ∠AFB is:
