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Math Help - external tangent to two circles

  1. #1
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    external tangent to two circles

    Hi ! Glad I've found this forum. I've been having a headache with this question.

    Find the length of the common external tangent to two circles of radii 5 cm and 12 cm, respectively if the distance between their centers is 25 cm.

    According to the textbook, the answer is 24 cm. I am getting an answer of 27 by basic trigo functions. Can any genius out there solve this ?
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  2. #2
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    Quote Originally Posted by redpilot1234 View Post
    Hi ! Glad I've found this forum. I've been having a headache with this question.

    Find the length of the common external tangent to two circles of radii 5 cm and 12 cm, respectively if the distance between their centers is 25 cm.

    According to the textbook, the answer is 24 cm. I am getting an answer of 27 by basic trigo functions. Can any genius out there solve this ?
    1. The bad news first: The book is right.

    2. You don't need any trigonometric functions but simple Pythagorean theorem: The length of the tangent is one leg of a right triangle whose hypotenuse is 25 and whose first leg is 12 - 5 = 7.
    Attached Thumbnails Attached Thumbnails external tangent to two circles-aussentangenten.png  
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  3. #3
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    Wow. Nice presentation ! So you're one of the geniuses here ... btw, what software did you use to draw that ? It would definitely help me in analyzing problems. Thanks !!!
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