UNIT 2
Text One
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+ 4 > 0 - 5 < 0 + 6 > 0 - 7 < 0 a > b b < a
+ 1 > 0
- 2 > 0 |
Plus four is greater than zero. Minus five is less then zero. Plus six is greater than zero. Minus seven is less than zero. a is greater than b. b is less than a. If a is greater than b, then b is less than a. «Plus one is greater than zero» is a sentence. It is a true sentence. «Minus two is greater than zero» is a sentence. It is a false sentence. If a sentence is true, then it is not false. If a sentence is false, then it is not rue. |
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A true sentence is a sentence which is not false. A false sentence is a sentence which is not true. |
Exercise: True or false?
1. Minus five is greater than zero.
2. plus three is less than zero.
3. Minus seven is greater than minus six.
4. Plus five is less than plus seven.
Text Two
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A right angle. A right angle is an angle which measures 90°.
An acute angle. An acute angle is an angle whose measure is more than 0° but less than 90°.
An obtuse angle. An obtuse angle is an angle which measures more than 90° and less than 180°.
ABC is a triangle. It is an acute triangle because all its angles are acute. DEF is a right triangle because one of its angles is a right angle. GHI is an obtuse triangle because one of its angles is an obtuse angle. |
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In any triangle the sum of the angles is equal to two right angles, or 180°. |
Exercise: True or false?
1. An obtuse triangle has one obtuse angle and two acute angles.
2. The sum of the angles of a triangle is equal to 90°.
3. An obtuse triangle has two obtuse angles.
4. A right triangle has one angle whose measure is equal to 90°.