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Two circles meet at points 
X and 
Y. Line segment 
[AXB] meets one circle at 
A and the other at 
B. Line segment [CYD] meets one circle at 
C and the other at 
D. Prove that 
[AC] is parallel to 
[BD].

Two circles meet at points X \mathrm{X} and Y \mathrm{Y} . Line segment [AXB] [\mathrm{AXB}] meets one circle at A \mathrm{A} and the other at B \mathrm{B} . Line segment [CYD] meets one circle at C C and the other at D D . Prove that [AC] [A C] is parallel to [BD] [B D] .

Full solution

Q. Two circles meet at points X \mathrm{X} and Y \mathrm{Y} . Line segment [AXB] [\mathrm{AXB}] meets one circle at A \mathrm{A} and the other at B \mathrm{B} . Line segment [CYD] meets one circle at C C and the other at D D . Prove that [AC] [A C] is parallel to [BD] [B D] .
  1. Identify Given Information: Identify the given information and the theorem that can be applied.\newlineIn this case, we are dealing with two circles intersecting at points XX and YY. The line segments [AXB][AXB] and [CYD][CYD] intersect the circles at points AA, BB, CC, and DD respectively. To prove that [AC][AC] is parallel to [BD][BD], we can use the concept of alternate segment theorem which states that the angle between the tangent and chord at the point of contact is equal to the angle in the alternate segment.
  2. Apply Alternate Segment Theorem: Apply the alternate segment theorem to the angles formed at points AA and BB. Since [AXB][AXB] is a line segment intersecting the circles at AA and BB, angle AXBAXB is subtended by arc XYXY in one circle, and angle AYBAYB is subtended by arc XYXY in the other circle. According to the alternate segment theorem, angle AXBAXB is equal to angle BB00, and angle AYBAYB is equal to angle BB22.
  3. Apply Theorem to Angles: Apply the alternate segment theorem to the angles formed at points CC and DD. Similarly, since [CYD][CYD] is a line segment intersecting the circles at CC and DD, angle CYDCYD is subtended by arc XYXY in one circle, and angle CXDCXD is subtended by arc XYXY in the other circle. According to the alternate segment theorem, angle CYDCYD is equal to angle DD00, and angle CXDCXD is equal to angle DD22.
  4. Combine Results for Parallel Lines: Combine the results from the alternate segment theorem to show parallel lines.\newlineFrom the previous steps, we have ADB=AXB\angle ADB = \angle AXB and ACB=AYB\angle ACB = \angle AYB. Also, CAD=CYD\angle CAD = \angle CYD and CBD=CXD\angle CBD = \angle CXD. Since angles AXBAXB and AYBAYB are the same, and angles CYDCYD and CXDCXD are the same, it follows that ADB=ACB\angle ADB = \angle ACB and CAD=CBD\angle CAD = \angle CBD. This means that angles ACB=AYB\angle ACB = \angle AYB00 and ACB=AYB\angle ACB = \angle AYB11 are alternate interior angles, and angles ACB=AYB\angle ACB = \angle AYB22 and ACB=AYB\angle ACB = \angle AYB33 are alternate interior angles.
  5. Conclude Parallel Lines: Conclude that [AC][AC] is parallel to [BD][BD]. If alternate interior angles are equal, then the lines are parallel by the converse of the alternate interior angle theorem. Therefore, since angle ADB=ACBADB = ACB and angle CAD=CBDCAD = CBD, line segment [AC][AC] is parallel to line segment [BD][BD].

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