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| Angles and Types of Angles |
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| When two straight lines meet at a common point, they form an angle. |
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 and  are called arms of the angle BAC. The point 'A' is called the vertex. The angle formed by the two rays AB and AC is denoted by the symbol  BAC. If there is only one angle at A, then the angle BAC may be denoted by  |
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| In a given plane two lines intersect at only one point. The angles are measured with the help of a protractor. The measure of an angle is called its magnitude. |
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| An angle whose measure is less than 90o is called an acute angle. |
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| It is an angle whose measure is equal to 90o. |
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| An angle whose measure is greater than 90o but less than 180o is called an obtuse angle. |
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| An angle with measure equal to 180o is called a straight angle. |
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| It is an angle with measure greater than 180o but less than 360o. |
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| Adjacent angles |
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| A pair of angles having a common arm and a common vertex. |
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 and  are adjacent angles. |
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| OB is the common arm. |
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| O is the common vertex. |
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| Complementary angles |
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| They are a pair of angles, the sum of whose measures is equal to 90o. These may be adjacent angles as well. |
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| (These are adjacent angles) |
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| (These are not adjacent angles) |
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| Supplementary angles |
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| They are a pair of angles, the sum of whose measures is equal to 180o. These may be adjacent angles as well. |
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 [See figure below] |
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 [See figure below] |
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| (non-adjacent angles) |
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| Linear pair of adjacent supplementary angles |
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| If a pair of angles are adjacent as well as supplementary then they are said to form a linear pair. |
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 are adjacent angles, |
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and  [See figure below] |
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