Section 1.2 Linear Measure and Precision
­ Units of measure give us points of reference when
evaluating the sizes of objects.
­ Unlike a line, a line segment, can be measured because it has two endpoints.
­ A segment with endpoints A and B can be named as AB or BA.
A
B
­ The length or measure of AB is written as AB.
­ The length of a segment is only as precise as the
smallest unit on the measuring device.
Ex 1 Find the length of CD using each ruler.
a.
b.
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Ex Ex 2 Find the length of AB using each ruler.
a.
b.
­ The precision of any measurement depends on the
smallest unit available on the measuring tool.
­ The measurement should be precise to within .5 unit
of measure.
­ For example, 3 centimeters means that the actual
length is no less than 2.5 centimeters, but no more
than 3.5 centimeters.
­ Measurements of 28 centimeters and 28.0
centimeters indicate different precision in
measurement.
­ A measurement of 28 centimeters means that the
ruler is divided into centimeters.
­ However, a measurement of 28.0 centimeters
indicates that the ruler is divided into millimeters.
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Ex 3 Find the precision for each measurement. Explain its meaning.
a. 5 millimeters (Hint: The measurement is
precise to within .5 millimeter.
b. inches
c. 15 centimeters
d. ft
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­ In geometry, the length of the whole line segment
is equal to the sum of the lengths of the parts of
the segment.
BETWEENNESS OF POINTS
Point M is between points P and Q if and only if P, Q, and M are collinear and Ex: Ex 4 Find the missing measure.
a. Find DE. b. Find LM.
c. Find XZ.
d. Find x and ST if T is between S and U, ST=7x, SU=45, and TU=5x­3.
e. Find y and PQ if P is between Q and R, PQ=2y, QR=3y+1, and PR=21.
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­ Look at the figure below. Notice that AB and BC
have the same measure.
­ When segments have the same measure, they
are said to be congruent.
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­ Constructions are methods of creating geometric
figures without the benefit of measuring tools,
such as a ruler.
­ Generally, only a pencil, straightedge, and
compass are used in constructions.
­ You can construct a segment that is congruent to
a given segment by using a compass and
straightedge.
Ex 5 In the graph below, suppose a segment was drawn along the top of each bar. Which categories would have segments that are congruent? Explain.
Assign Pgs. 16 ­ 19 # 9 ­ 19, 21 ­ 39, 41, 46 ­ 49,
58 ­ 61
Pg. 19 # 1 ­ 5
Homework Notes: Do # 9, 10, 33 with them
Explain how to do # 48, 49
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