6th Grade Math Textbook, Fundamentals

You can write zeros to help align

the digits in each place-value column.

 &KDSWHU

4-5

Add: 3.456 (2.3)

First estimate by rounding:  3 (2)  5

Then add.

5.756 is close to the estimate of 5, so the answer is reasonable.

So 3.456 (2.3) 5.756.

3.456

2.300

5.756

|3.456 |

|2.300 |

3.456

(2.300)

1

Key Concept

Add Decimals with Unlike Signs

• Find the absolute value of each addend.


• Subtract the lesser absolute value
  from the greater absolute value.


• Use the sign of the addend with the
  greater absolute value for the sum.

1.55 is close to the estimate of 2,

so the answer is reasonable.

Key Concept

Add Decimals with Like Signs

• Add the absolute values.


• Use the sign of the addends
  for the sum.

Subtraction Principle

To subtract a rational number, add its opposite.

aba(b)

To subtract positive and negative decimals,

use the rule for subtracting integers, also

called theSubtraction Principle.

Update your skills. See page 409 VI.

Add and Subtract Decimals

Objective To add and subtract positive and negative decimals • To add more than two decimals

Over its first six months, a company posted a loss of

$1.05 million. Over its second six months, it posted a

gain of $2.6 million. In millions of dollars, what was

the company’s net gain or loss for its first year?

Let nequal the net gain or loss.

n 1.05 2.6

You add decimals with unlikesigns the same

way that you add integers with unlike signs.

First estimate by rounding: n 1 3, or 2

Then add: n1.05 2.6

Subtract the lesser absolute value from the

greater absolute value.

2.60

1.05

1.55 |2.6||1.05|, so the sum is positive.

n1.55

So the loss of $1.05 million and the gain of $2.6 million

created a net gain of $1.55 million.

Check: Use a calculator to check the answer.

1.05 2.6

To add decimals with likesigns, align place values,

and add the same way that you add integers.

A loss of 1.05 is represented by 1.05, and

a gain of 2.6 is represented by 2.6.

1.55

Both addends are negative,

so the sum is negative.