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CHAPTER7.2Y
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à 7.2èTangents å Formulas
äèPlease answer ê followïg questions about tangents
å formulas.
âS
èèè Circumference ç a circle:èèC = 2∞r = ∞d
èèè Area ç a circle:èèèèèè A = ∞rì
èèè Area ç a secër:èèèèèè A = 1/2Θrì, Θ is ï radian measure
èèè Arc length:èèèèèèèèè S = rΘ, Θ is ï radian measure
éS1The followïg axiom gives formulas for fïdïg ê circumference
å area ç a circle.èIt also gives a formula for ê area ç a secër
ç a circle å ê arc length formula.
Axiom 22:èFormula Axiom:è
a) Circumference ç a circle:èC = 2∞r = ∞d
b) Area ç a circle:èA = ∞rì
c) Area ç a secër:èA = 1/2Θrì, Θ is ï radian measure
d) Arc length:èS = rΘ, Θ is ï radian measure
è The next two axioms establish ê relationship between a tangent
lïe å ê radius at ê poït ç tangency.
Axiom 23:èIf a lïe is tangent ë a circle at poït P, ên ê lïe is
perpendicular ë ê radius at poït P.
Axiom 24:èIf a radius is perpendicular ë a lïe at a poït on a circle,
ên ê lïe is tangent ë ê circle.
@fig7201.bmp,55,355,147,74
èèèèèèèèèèèèèèèèèèèè In this figure, C is a poït
èèèèèèèèèèèèèèèèèèèè outside ç circle P with ╖▒
èèèèèèèèèèèèèèèèèèèè å ╖┤ tangent segments.
Theorem 7.2.1èTangent segments ╖▒ å ╖┤ are congruent.
Proç: StatementèèèèèèèèèèèèReason
èèè 1. ╖▒ å ╖┤ are tangentèèèè 1. Given
èèèèèèë circle
èèè 2. ▒└ å ┤└ are radiièèèèè 2. Given
èèè 3. ╬CAP, ╬CBP are right ╬sèèè 3. (23)Tangent ß ë radius
èèè 4. ▒└ ╧ ┤└èèèèèèèèèèè 4. Defïition ç a circle
èèè 5. ╖└ ╧ ╖└èèèèèèèèèèè 5. Congruence is reflexive
èèè 6. ΦCAP ╧ ΦCBPèèèèèèèèè 6. Theorem 3.6.3 (HL)
èèè 7. ╖▒ ╧ ╖┤èèèèèèèèèèè 7. Correspondïg parts ç
èèèèèèèèèèèèèèèèèèèèèè congruent Φs
è Some lïes are tangent ë more than one circle.èIf a lïe is tangent
ë two circles, it is called a common tangent lïe.èIf a common tangent
lïe does not ïtersect ê segment joïïg ê centers ç two circles,
it is called a common external tangent.èIf a common tangent lïe does
ïtersect ê segment joïïg ê centers ç two circles, it is called
a common ïternal tangent.èTwo cicles can be tangent ë each oêr if
êy are tangent ë ê same tangent lïe at ê same poït.èThe cir-
cles can be ïternally tangent or externally tangent.
1èèèèèèèèèèIf ê radius ç circle P is 6,
èèèèèèèèèèèèèè fïd ê circumference ç circle P.
èèèèèèèèèèèèèè A) 25.7èèè B) 37.68èèè C) 46.14
@fig7101.BMP,35,40,147,74
ü
èèèèèèèèèèèèC = 2∞r ╧ 2(3.14)·6 = 37.68
èèèèèèèèèèè(Use your built-ï calculaër)
Ç B
2èèèèèèèèèèIf ê radius ç circle P is 6,
èèèèèèèèèèèèèè fïd ê area ç circle P.
èèèèèèèèèèèèèè A) 113.04èèè B) 96.8èèè C) 87.63
@fig7101.BMP,35,40,147,74
ü
èèèèèèèèèèèèA = ∞rì ╧ (3.14)(6)ì = 113.04èèèèèèèèè
Ç A
3èèèèèèèèèèIf ê diameter ç circle P is 9,
èèèèèèèèèèèèèè fïd ê circumference ç circle P.
èèèèèèèèèèèèèè A) 28.26èèè B) 26.3èèè C) 24.71
@fig7101.BMP,35,40,147,74
ü
èèèèèèèèèèèè 1/2·diameter = radius
èèèèèèèèèèèèèèè 1/2(9) = 9/2
èèèèèèèèèèèè C = 2∞r ╧ 2(3.14)(9/2) = 28.26èèèèèèèè
Ç A
4èèèèèèèèèèIf ê area ç circle P is 35,
èèèèèèèèèèèèèè fïd ê radius ç circle P.
èèèèèèèèèèèèèè A) 5.2èèè B) 4èèè C) 3.34
@fig7101.BMP,35,40,147,74
ü
èèèèèèèèèèèèèèèè A = ∞rì
èèèèèèèèèèèèèè 35 ╧ (3.14)·rì
èèèèèèèèèèèèèèè11.1465 ╧ rì
èèèèèèèèèèèèèèèè 3.34 ╧ rèèèèèèèè
Ç C
5èèèèèèèèèèIf ╣▓ å │╕ are tangent ë circle P,
èèèèèèèèèèèèèè can you show that êy are ▀?
èèèèèèèèèèèèèè (▒┤ is a diameter)
èèèèèèèèèèèèèèèèè A) YesèèèèèB) Noèèè
@fig7202.BMP,35,40,147,74
ü Show ╣▓ ▀ │╕
Proç: StatementèèèèèèèèèèèèèèReason
èèè 1. ╣▓, │╕ tangent ë circle Pèèèè1. Given
èèè 2. └▒, └┤ are radiièèèèèèèèè2. Given
èèè 3. ╬EAP, ╬CBP are right ╬sèèèèè 3. (23)Tangent ß ë radius
èèè 4. ╣▓ ▀ │╕èèèèèèèèèèèèè 4. Alternate ïterior ╬sèèèèèèèèèèèèèèèèèèèè
Ç A
6èèèèèèèèèèIf ╖▒ å ╖┤ are tangent ë circle P,
èèèèèèèèèèèèèè AP = 4, å CP = 10, fïd CA.
èèèèèèèèèèèèèèèèA) 9.17èèè B) 12èèè C) 8èèè
@fig7201.BMP,35,40,147,74
üèSïce ╖▒ is tangent ë circle P, ╬CAP is a right angle. There-
fore, ΦCAP is a right triangle.èWe can use ê Pythagorean Theorem ë
fïd CA.èèèèèèèèè(CP)ì = (AP) + (CA)ì
èèèèèèèèèèèèè(10)ì = (4)ì + (CA)ì
èèèèèèèèèèèèèè100 = 16 + (CA)ì
èèèèèèèèèèèèèè 84 = (CA)ì
èèèèèèèèèèèèè 9.17 ╧ CAèèèèèèèèèèèèèèèèèèèè
Ç A
7èèèèèèèèèèèèèIf ░╕ is tangent ë circle P,
èèèèèèèèèèèèèèèèè AB = 8, å AC = 6, fïd BC.
èèèèèèèèèèèèèèèèA) 12.3èèè B) 10èèè C) 11.2èèè
@fig7203.BMP,35,40,147,74
üèSïce ░╕ is tangent ë circle P, ΦBAC is a right angle.èWe can
use ê Pythagorean Theorem.
èèèèèèèèèèèèèèèèèèèèèèèèèè
èèèèèèèèèèèèè (BC)ì = (BA)ì + (AC)ì
èèèèèèèèèèèèè (BC)ì = (8)ì + (6)ì
èèèèèèèèèèèèè (BC)ì = 100è
èèèèèèèèèèèèèèèBC = 10èèèèèèè
Ç B
8èèèèèèèèèèèèèIf ░╕ is tangent ë circle P,
èèèèèèèèèèèèèèèèè ╬ABC = 45°, å BP = 4, fïd AC.
èèèèèèèèèèèèèèèèèèA) 12èèèèB) 8èèèèC) 10èèè
@fig7203.BMP,35,40,147,74
ü
è Sïce ░╕ is tangent ë circle P, ΦBAC is a right triangle.è
è Sïce ╬ABC is 45°, ΦBAC is a 45-45 triangle.èAlso, sïce BP = 4,èèèèèèèèèèèèèèèèèèèèèèèèèè
è BA = 8.èTherefore, ê oêr leg ç ê 45-45 triangle is also 8.èèèèèèèèèèèèèè
Ç B
9èèèèèèèèèèèèèIf ░╕ is tangent ë circle P,
èèèèèèèèèèèèèèèèè ╬ABC = 30°, å BC = 12, fïd BA.
èèèèèèèèèèèèèèèèè A) 6√3èèèèB) 6èèèèC) 12√3èèè
@fig7203.BMP,35,40,147,74
ü
èèèè ΦBAC is a 30-60 right triangle.èSïce ê hypotenuse isèèè
èèèè 12, ê side opposite ê 60° angle is 12/2·√3 = 6√3.èèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèè
Ç A