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3. The distance between the centers of the two circles in Figure 10, having radii
of 3 and 6, is 18. How long is the common internal tangent?​


Sagot :

✒️CIRCLE

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[tex] \large\underline{\mathbb{PROBLEM}:} [/tex]

  • The distance between the centers of the two circles in Figure 10, having radii
  • of 3 and 6, is 18. How long is the common internal tangent?

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[tex] \large\underline{\mathbb{ANSWER}:} [/tex]

[tex] \qquad \LARGE \:\: \rm{9\sqrt3 \: units} [/tex]

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[tex] \large\underline{\mathbb{SOLUTION}:} [/tex]

Illustration:

  • Label the points A and B as the centers of the two circles. Draw a segment between them measuring 18 units.

  • Draw one common internal tangent labeled as DC.

  • From their point of tangency (D and C), draw radii AD and BC then indicate their measures.

  • Label intersection of AB and DC as F.

Solving:

» By the construction, we have created two similar right triangles.

  • [tex] \Delta ADF \: \sim \Delta BCF [/tex]

» And then, we can find the proportion of the sides of these triangles as:

  • [tex] \frac{\,AD\,}{BC} = \frac{\,AF\,}{B F} \\ [/tex]

» Let x be the measure of AF and (18-x) for B F since AB is 18. Find x.

  • [tex] \frac{\,3\,}{6} = \frac{x}{\,18-x\,} \\ [/tex]

  • [tex] \frac{\,1\,}{2} = \frac{x}{\,18-x\,} \\ [/tex]

  • [tex] 1(18 - x) = 2(x) [/tex]

  • [tex] 18 - x = 2x [/tex]

  • [tex] 2x + x = 18 [/tex]

  • [tex] 3x = 18 [/tex]

  • [tex] \frac{\,3x\,}{3} = \frac{\,18\,}{3} \\ [/tex]

  • [tex] x = 6 [/tex]

» Thus, AF measures 6 units. Solve for DF using the Pythagorean Theorem.

  • [tex] (DF)^2 + (AD)^2 = (AF)^2 [/tex]

  • [tex] (DF)^2 + (3)^2 = (6)^2 [/tex]

  • [tex] (DF)^2 + 9 = 36 [/tex]

  • [tex] (DF)^2 = 36 - 9 [/tex]

  • [tex] (DF)^2 = 27 [/tex]

  • [tex] \sqrt{(DF)^2} = \sqrt{27} [/tex]

  • [tex] DF = 3\sqrt3 [/tex]

» Thus, DF measures 3√3 units. Find CF using proportion.

  • [tex] \frac{\,AD\,}{BC} = \frac{\,DF\,}{CF} \\ [/tex]

  • [tex] \frac{\,3\,}{6} = \frac{\,3\sqrt3\,}{CF} \\ [/tex]

  • [tex] \frac{\,1\,}{2} = \frac{\,3\sqrt3\,}{CF} \\ [/tex]

  • [tex] 1(CF) = 2(3\sqrt3) \\ [/tex]

  • [tex] CF = 6\sqrt3 \\ [/tex]

» Thus, CF measures 6√3 units. Find the measure of the common internal tangent or the measure of DC by the sum of DF and CF.

  • [tex] DC = DF + CF [/tex]

  • [tex] DC = 3\sqrt3 + 6\sqrt3 [/tex]

  • [tex] DC = 9\sqrt3 [/tex]

[tex] \therefore [/tex] The measure of the common internal tangent of the two circles is 93 units.

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