1. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Lesson 5-3 Quiz – Concurrent Lines, Medians, and Altitudes
1. Complete the sentence: To find the centroid of a triangle,
you need to draw at least ? median(s).
FGH has vertices F(–1, 2), G(9, 2), and H(9, 0). Find the
center of the circle that circumscribes FGH.
Use the diagram for Exercises 3–5.
3. Identify all medians and altitudes drawn in PSV.
4. If SY = 15, find SM and MY.
5. If MX = 14, find PM and PX.
two
(4, 1)
PX and SY are medians; VZ is an altitude.
SM = 10 and MY = 5
PM = 28 and PX = 42
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5. Remember! Inverses, Contrapositives, and Indirect Reasoning Lesson 5-4

6. The negation of statement p is “not p.”
Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Conditional and Related Statements
Definition
Symbols
A conditional statement is a statement that can be written in the form “if p, then q.”
The converse is the statement formed by exchanging the hypothesis and the conclusion.
The negation of statement p is “not p.”
The inverse is the statement formed by negating the hypothesis and conclusion.
The contrapositive is the statement formed by both exchanging and negating the hypothesis and conclusion.
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7. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
The negation of a true statement is false, and the negation of a false statement is true.
Remember!
A conditional statement has a truth value of either true (T) or false (F). It is false only when the hypothesis is true and the conclusion is false.
To show that a conditional statement is false, you need to find only one counterexample where the hypothesis is true and the conclusion is false.
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8. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Related conditional statements that have the same truth value are called logically equivalent statements. A conditional and its contrapositive are logically equivalent, and so are the converse and inverse.
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9. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
The logical equivalence of a conditional and its contrapositive is known as the Law of Contrapositive.
Helpful Hint
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10. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
So far you have written proofs using direct reasoning. You began with a true hypothesis and built a logical argument to show that a conclusion was true.
In an indirect proof, you begin by assuming that the conclusion is false. Then you show that this assumption leads to a contradiction. This type of proof is also called a proof by contradiction.
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11. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
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12. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
When writing an indirect proof, look for a contradiction of one of the following: the given information, a definition, a postulate, or a
theorem.
Helpful Hint
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13. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Additional Examples
Writing the Negation of a Statement
Write the negation of “ABCD is not a convex polygon.”
The negation of a statement has the opposite truth value. The
negation of is not in the original statement removes the word not.
The negation of “ABCD is not a convex polygon” is “ABCD is a convex polygon.”
Quick Check
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14. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Additional Examples
Writing the Inverse and Contrapositive
Quick Check
Write the inverse and contrapositive of the conditional statement “If ABC is equilateral, then it is isosceles.”
To write the inverse of a conditional, negate both the hypothesis and
the conclusion.
Hypothesis Conclusion
Conditional: If ABC is equilateral, then it is isosceles.
Negate both.
Inverse: If ABC is not equilateral, then it is not isosceles.
To write the contrapositive of a conditional, switch the hypothesis and
conclusion, then negate both.
Conditional: If ABC is equilateral, then it is isosceles.
Switch and negate both.
Contrapositive: If ABC is not isosceles, then it is not equilateral.
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15. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Additional Examples
The First Step of an Indirect Proof
Write the first step of an indirect proof.
Prove: A triangle cannot contain two right angles.
In the first step of an indirect proof, you assume as true the negation
of what you want to prove.
Because you want to prove that a triangle cannot contain two right
angles, you assume that a triangle can contain two right angles.
The first step is “Assume that a triangle contains two right angles.”
Quick Check
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16. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Additional Examples
Identifying Contradictions
Quick Check
Identify the two statements that contradict each other.
I. P, Q, and R are coplanar.
II. P, Q, and R are collinear.
III. m PQR = 60
Two statements contradict each other
when they cannot both be true
at the same time.
Examine each pair of statements to see whether they contradict each other.
I and II
P, Q, and R are
coplanar and
collinear.
Three points that lie
on the same line are
both coplanar and
collinear, so these
two statements do
not contradict each
other.
I and III
coplanar, and
m PQR = 60.
on an angle are
coplanar, so these
II and III
collinear, and
m PQR = 60.
If three distinct
points are collinear,
they form a straight
angle, so m PQR
cannot equal 60.
Statements II and III
contradict each
I and II
P, Q, and R are
coplanar and
collinear.
Three points that lie
on the same line are
both coplanar and
collinear, so these
two statements do
not contradict each
other.
I and II
P, Q, and R are
coplanar and
collinear.
Three points that lie
on the same line are
both coplanar and
collinear, so these
two statements do
not contradict each
other.
I and III
coplanar, and
m PQR = 60.
on an angle are
coplanar, so these
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17. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Additional Examples
Indirect Proof
Write an indirect proof.
Prove: ABC cannot contain 2 obtuse angles.
Assume that ABC contains two obtuse angles. Let  A and B be obtuse.
If A and B are obtuse, mA > 90 and mB > 90, so mA + mB > 180.
Because m C > 0, this means that m A + m B + m C > 180.
This contradicts the Triangle Angle-Sum Theorem, which states
that m A + m B + m C = 180.
The assumption in Step 1 must be false.
ABC cannot contain 2 obtuse angles.
Quick Check
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18. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Lesson Quiz
1. Write the negation of the statement “ D is a straight angle.”
2. Identify two statements that contradict each other.
I. x and y are perfect squares.
II. x and y are odd.
III. x and y are prime.
For Exercises 3–6, use the following statement:
If is parallel to m, then and are supplementary.
3. Write the converse.
4. Write the inverse.
5. Write the contrapositive.
6. Write the first step of an indirect proof.
D is not a straight angle.
I and III
If and are supplementary, then is parallel to m.
If is not parallel to m, then and are not supplementary.
If and are not supplementary, then is not parallel to m.
Assume that and are not supplementary.
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19. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Check Skills You’ll Need
1. If it snows tomorrow, then we will go skiing.
2. If two lines are parallel, then they do not intersect.
3. If x = –1, then x2 = 1.
Write two conditional statements that make up each biconditional.
4. A point is on the bisector of an angle if and only if it is equidistant from
the sides of the angle.
5. A point is on the perpendicular bisector of a segment if and
only if it is equidistant from the endpoints of the segment.
6. You will pass a geometry course if and only if you are successful with your homework.
(For help, go to Lessons 2-1 and 2-2.)
Write the converse of each statement.
Check Skills You’ll Need
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20. Inverses, Contrapositives, and Indirect Reasoning
Lesson 5-4
Check Skills You’ll Need
Solutions
1. Switch the hypothesis and conclusion: If we go skiing tomorrow, then it snows.
2. Switch the hypothesis and conclusion: If two lines do not intersect, then they
are parallel.
3. If x2 = 1, then x = –1.
4. Rewrite the statement as an if-then statement; then rewrite it by writing
its converse: If a point is on the bisector of an angle, then it is equidistant from
the sides of the angle. If a point is equidistant from the sides of an angle,
then it is on the bisector of the angle.
5. Rewrite the statement as an if-then statement; then rewrite it by writing
its converse: If a point is on the perpendicular bisector of a segment, then
it is equidistant from the endpoints of the segment. If a point is equidistant
from the endpoints of a segment, then it is on the perpendicular bisector
of the segment.
6. Rewrite the statement as an if-then statement; then rewrite it by writing its
converse: If you pass a geometry course, then you are successful with your
geometry homework. If you are successful with your geometry homework,
then you will pass the geometry course.
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