Geometry · Learn by Concept

Geometry Help: Constructing Tangents To Circles

Review real example questions for Constructing Tangents To Circles in Geometry.

Question 1 / 10

0 of 10 answered

A line $p$ is tangent to circle $\odot Q$ at point J. Segment $QJ$ is drawn, and the right angle between $QJ$ and $p$ at $J$ is marked. Which reasoning correctly uses the radius–tangent relationship?

All questions

Question 1

Because $p$ is tangent at $J$ , $QJ \perp p$ at $J$ . (correct answer)
Because $p$ is tangent at $J$ , $QJ \parallel p$ .
Because $QJ$ is a radius, line $p$ meets the circle at two points.
Because $QJ$ is a radius, point $J$ must be the center.

Explanation: This question explores tangent properties in circle geometry. A tangent to a circle is a line that contacts the circle at precisely one point. This point is the point of tangency, labeled J. The radius QJ is perpendicular to the tangent p at J, forming the marked right angle. This reasoning correctly applies the radius-tangent perpendicularity theorem. A distractor like choice B incorrectly claims parallelism instead of perpendicularity. In solving, always connect the center to the tangent point and apply the perpendicular property.

Question 2

Circle $\odot M$ has tangent line $n$ touching it at point Q. The radius $MQ$ is drawn, and the right angle between $MQ$ and $n$ at $Q$ is marked. Which reasoning correctly uses the radius–tangent relationship?

Since $n$ touches the circle at $Q$ , $MQ$ must be perpendicular to $n$ . (correct answer)
Since $MQ$ is a radius, it must be parallel to tangent $n$ .
Since $n$ is a tangent, it must cross the circle at two points.
Since $Q$ is on the circle, $MQ$ must be a chord.

Explanation: This problem involves properties of tangents to circles. A tangent to a circle is a line that touches the circle at exactly one point. The point where the tangent touches the circle is called the point of tangency, here labeled as Q. At the point of tangency, the radius drawn from the center M to Q is perpendicular to the tangent line n. This perpendicularity correctly uses the radius-tangent relationship, justifying the reasoning. A common misconception is that the radius is parallel to the tangent, but it is perpendicular. To solve similar problems, always find the radius to the tangent point and apply the perpendicular property.

Question 3

Using compass and straightedge, a student constructs tangent lines from external point $E$ to circle $F$ by first drawing an auxiliary circle. The construction is successful, yielding two intersection points that determine the tangent lines. Which statement must be true about the auxiliary circle used in this construction?

The auxiliary circle has the same radius as the original circle and is centered at point $E$ to ensure proper intersection angles
The auxiliary circle is concentric with the original circle but has radius equal to the distance $EF$ to guarantee intersection
The auxiliary circle passes through both point $E$ and center $F$ , with its center located at the midpoint of segment $EF$ (correct answer)
The auxiliary circle has center at point $E$ and radius equal to the distance $EF$ to create the proper geometric relationship

Explanation: When you encounter questions about compass and straightedge constructions for tangent lines from an external point, focus on the geometric properties that make the construction work. The key insight is understanding what auxiliary circle creates the right conditions for finding tangent points. The correct construction uses an auxiliary circle that passes through both the external point $E$ and the center $F$ of the original circle, with its center at the midpoint of segment $EF$ . This creates a semicircle where $EF$ is a diameter. When this auxiliary circle intersects the original circle, it produces two crucial intersection points. By the inscribed angle theorem, any angle inscribed in a semicircle is a right angle. This means the lines from these intersection points to point $E$ are perpendicular to the radii of the original circle at those points—which is precisely the definition of a tangent line. Choice A is incorrect because having the same radius and centering at $E$ doesn't create the perpendicular relationship needed for tangency. Choice B fails because being concentric (same center) with radius $EF$ would place the auxiliary circle's center at $F$ , not creating the necessary geometric configuration. Choice D places the center at $E$ with radius $EF$ , but this doesn't establish the right angle property required for tangent lines. Remember this pattern: geometric constructions often rely on creating specific angle relationships. When you see tangent line constructions, look for methods that guarantee perpendicularity between the tangent and radius.

Question 4

During a compass and straightedge construction of tangent lines from external point $P$ to circle $O$ , a student draws the auxiliary circle with diameter $PO$ but finds it doesn't intersect the original circle. What error did the student most likely make?

The compass opening was set incorrectly when drawing the original circle, making its radius too large for the construction
Point $P$ was actually chosen inside the original circle, making the auxiliary circle too small to reach the original circle (correct answer)
The midpoint of segment $PO$ was located incorrectly, causing the auxiliary circle to be centered at the wrong position
The auxiliary circle was drawn with $PO$ as a chord rather than a diameter, resulting in a circle too small to intersect

Explanation: For the standard tangent construction to work, point $P$ must be outside the original circle. The auxiliary circle has diameter $PO$ (where $O$ is the center of the original circle), so its radius is $\frac{PO}{2}$ and it's centered at the midpoint of $PO$ . If $P$ is outside the original circle with radius $r$ , then $PO > r$ . The auxiliary circle extends from the midpoint toward $O$ by distance $\frac{PO}{2}$ , and since $\frac{PO}{2} > \frac{r}{1} = r$ when $PO > 2r$ , it will intersect the original circle. However, if $P$ is inside the original circle, then $PO < r$ , making the auxiliary circle's radius $\frac{PO}{2} < \frac{r}{2}$ , and it cannot reach the original circle. Choice A is incorrect because the original circle's size doesn't affect intersection. Choice C is wrong because an incorrectly centered auxiliary circle would still likely intersect if properly sized. Choice D is incorrect because using $PO$ as a chord (not diameter) would create a larger circle, not smaller.

Question 5

A circle $\odot O$ is shown with tangent line $\overleftrightarrow{\ell}$ touching the circle at point S. The radius $\overline{OS}$ is drawn, and the right angle at $S$ is marked between $\overline{OS}$ and $\ell$ . Which conclusion is NOT justified?

$\overline{OS} \perp \overleftrightarrow{\ell}$ at $S$ .
Point $S$ lies on the circle.
Line $\ell$ intersects the circle only at $S$ .
Segment $\overline{OS}$ is a chord of the circle. (correct answer)

Explanation: This question asks which conclusion is NOT justified when dealing with tangents to circles. A tangent line touches a circle at exactly one point, and at this point of tangency, the tangent is perpendicular to the radius. Here, line ℓ is tangent to circle O at point S, which is the point of tangency. Since OS is a radius (connecting center O to point S on the circle), we can justify that OS is perpendicular to ℓ at S (choice A), that point S lies on the circle (choice B), and that line ℓ intersects the circle only at S (choice C). However, choice D claims that OS is a chord, which is incorrect—a chord connects two points on the circle, but OS connects the center to a point on the circle, making it a radius, not a chord. Students often confuse radii with chords; remember that all radii start at the center, while chords connect two points on the circle's circumference.

Question 6

Two circles have centers $A$ and $B$ respectively, with $AB = 10$ . Circle $A$ has radius $3$ and circle $B$ has radius $4$ . If a common external tangent line is drawn to both circles, what is the distance between the points where this tangent touches each circle?

$\sqrt{99}$ (correct answer)
$\sqrt{101}$
$\sqrt{91}$
$9$

Explanation: For two external circles with centers distance $d$ apart and radii $r_1$ and $r_2$ , a common external tangent creates a trapezoid where the parallel sides are the radii to the tangent points. The distance between tangent points can be found using coordinate geometry or by recognizing that if we drop a perpendicular from one tangent point to the line through the other center parallel to the common tangent, we form a right triangle. The horizontal distance between centers is $10$ , and the vertical separation needed is $|4-3| = 1$ (difference in radii). Using the Pythagorean theorem on the right triangle formed: $\text{distance}^2 = 10^2 - 1^2 = 100 - 1 = 99$ , so the distance is $\sqrt{99}$ . Choice B ( $\sqrt{101}$ ) would result from incorrectly adding the radii: $10^2 + 1^2$ . Choice C ( $\sqrt{91}$ ) might come from an error like $10^2 - 3^2$ . Choice D ( $9$ ) could result from simply subtracting: $10 - 1$ .

Question 7

Point $W$ lies outside circle $Z$ , and tangent segments $WA$ and $WB$ are drawn to the circle (with $A$ and $B$ being points of tangency). If the radius of circle $Z$ is $5$ and $WZ = 13$ , what is the perimeter of quadrilateral $WAZB$ ?

$24$
$44$
$36$
$34$ (correct answer)

Explanation: When you see tangent segments drawn from an external point to a circle, think about the key properties: tangent segments from the same external point are equal in length, and each tangent is perpendicular to the radius at the point of tangency. Since $WA$ and $WB$ are tangent segments from point $W$ to circle $Z$ , we know $WA = WB$ . To find these lengths, use the right triangles $WAZ$ and $WBZ$ . Each has a right angle where the tangent meets the radius ( $\angle WAZ = \angle WBZ = 90°$ ). In right triangle $WAZ$ : $WZ = 13$ (hypotenuse), $AZ = 5$ (radius), so by the Pythagorean theorem: $WA^2 + AZ^2 = WZ^2$ , which gives us $WA^2 + 25 = 169$ , so $WA^2 = 144$ and $WA = 12$ . Similarly, $WB = 12$ . The perimeter of quadrilateral $WAZB$ is $WA + AZ + ZB + BW = 12 + 5 + 5 + 12 = 34$ . Choice A ( $24$ ) likely comes from adding only the tangent segments: $12 + 12 = 24$ , forgetting the two radii. Choice B ( $44$ ) might result from incorrectly calculating the tangent length as $18$ instead of $12$ , then adding all four sides. Choice C ( $36$ ) could come from miscalculating the tangent segments as $13$ each, giving $13 + 5 + 5 + 13 = 36$ . Remember: tangent segments from an external point are always equal, and they form right angles with radii at the points of tangency—perfect setup for the Pythagorean theorem.

Question 8

A circle with center $O$ is drawn. A line $\ell$ is tangent to the circle at T, and radius $OT$ is drawn. The right angle between $OT$ and $\ell$ is marked at $T$ . Which reasoning correctly uses the radius–tangent relationship?

Since $OT$ is a radius, $OT$ must be parallel to the tangent $\ell$ .
Since $\ell$ touches the circle, it must pass through the center $O$ .
Since $\ell$ is tangent at $T$ , $OT$ is perpendicular to $\ell$ at $T$ . (correct answer)
Since $OT$ meets $\ell$ at $T$ , $\ell$ must cut the circle at two points.

Explanation: The skill involves understanding properties of tangents to circles. A tangent to a circle is a line that touches the circle at exactly one point. The point where the tangent touches the circle is called the point of tangency, here point T. At the point of tangency, the radius to that point is perpendicular to the tangent line. Therefore, since ℓ is tangent at T, OT is perpendicular to ℓ at T, correctly using the relationship. A common misconception is that the tangent must pass through the center, but it does not. To solve similar problems, identify the radius to the point of tangency and apply the perpendicular property.

Question 9

A point $P$ is outside circle $\odot O$ . Two tangents from $P$ touch the circle at points C and D. Radii $\overline{OC}$ and $\overline{OD}$ are drawn, and right angles are marked at $C$ and $D$ where each radius meets its tangent. Which statement must be true at the points of tangency?

$\overline{OC}$ and $\overline{OD}$ are chords of the circle.
Each tangent is perpendicular to its radius at the point of tangency. (correct answer)
Each tangent intersects the circle at two points.
The two tangents are parallel to each other.

Explanation: This question examines properties of tangent lines drawn from an external point to a circle. A tangent line touches a circle at exactly one point, and the fundamental property states that each tangent is perpendicular to the radius at its point of tangency. From external point P, two tangents are drawn touching the circle at points C and D. At each point of tangency, the radius (OC at point C, and OD at point D) is perpendicular to its respective tangent line, which is what choice B correctly states. Choice A incorrectly identifies radii OC and OD as chords—radii connect the center to points on the circle, while chords connect two points on the circle's circumference. Choice C wrongly claims tangents intersect at two points (they touch at exactly one). To work with tangents from external points, remember that each tangent maintains the perpendicular relationship with its radius at the point of tangency.

Question 10

Circle $\odot G$ is shown with tangent line $u$ touching the circle at point K. Radius $GK$ is drawn, and the right angle at $K$ between $GK$ and $u$ is marked. Which angle relationship is guaranteed?

$\angle G K u=180^\circ$ .
$\angle(GK,u)=90^\circ$ at $K$ . (correct answer)
$\angle G K u=45^\circ$ .
$\angle(GK,u)$ cannot be determined from the diagram.

Explanation: This problem evaluates tangent properties in circle geometry. A tangent to a circle is a line that meets the circle at precisely one point. This point is the point of tangency, labeled K. The radius GK is perpendicular to the tangent u at K, guaranteeing a right angle as marked. This relationship is ensured by the tangent-radius theorem. A misconception in choice A suggests a straight angle, ignoring the perpendicular property. In practice, find the radius to the tangent point to determine the right angle.