Add two decimals

### 1.1. Knowledge to remember

**a)** Example 1: A curved line ABC has a straight line segment AB 1.84m long and a line segment BC 2.45m long. How many meters long is that bend?

We have to do the addition:

1.84 + 2.45 = ? (m)

We have: 1.84m + 184 cm

2.45m = 145 cm

So 1.84 + 2.45 = 4.29 (m)

Usually we set the calculation and then do the following:

Perform addition operations like adding natural numbers.

Write commas in column sum with commas of terms

**b) **Example 2: 15.9 + 8.75 = ?

We calculate and then do the following:

Perform addition operations like adding natural numbers.

Write commas in column sum with commas of terms

To add two decimals, do the following:

– Write one term below the other such that the digits in the same row line up the column.

– Add like adding natural numbers số

– Write commas in the column sum with commas of terms

### 1.2 . Solution of textbook exercises page 50

**Lesson 1:**

Calculate:

\(\begin{array}{l}

a) + \underline {\begin{array}{*{20}{c}}

{58.2}\\

{24.3}

\end{array}} \\

b) + \underline {\begin{array}{*{20}{c}}

{19,36}\\

{4,08}

\end{array}} \\

c) + \underline {\begin{array}{*{20}{c}}

{75,8}\\

{249.19}

\end{array}} \\

d) + \underline {\begin{array}{*{20}{c}}

{0.995}\\

{0.868}

\end{array}}

\end{array}\)

*Solution guide:*

\(a)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{58.2}\\

{24.3}

\end{array}} }\\

{\,\,\,82,5}

\end{array}\)

\(b)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{19,36}\\

{4,08}

\end{array}} }\\

{23,44}

\end{array}\)

\(c)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{75,8}\\

{249,19}

\end{array}} }\\

{\,\,\,324,99}

\end{array}\)

\(d)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{0.995}\\

{0.868}

\end{array}} }\\

{\,\,\,1,863}

\end{array}\)

Lesson 2:

Calculate then calculate:

a) 7.8+9.6

b) 34.82+9.75

c) 57,648+35.37.

*Solution guide:*

\(a)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{7,8}\\

{9,6}

\end{array}} }\\

{17.4}

\end{array}\)

\(b)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{34,82}\\

{9.75}

\end{array}} }\\

{44,57}

\end{array}\)

\(c)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{57,648}\\

{35,37}

\end{array}} }\\

{\,\,\,\,\,93,018}

\end{array}\)

Lesson 3:

Male weight 32.6kg. Tien weighs 4.8kg more than Nam. How many kilograms does Tien weigh?

*Solution guide:*

The weight advance in kilograms is:

32.6 + 4.8 = 37.4(kg)

Answer: 37.4kg

### 1.3. Solving exercises in the Practice Textbook Pages 50, 51

Lesson 1:

Calculate and compare the values of a+b and b+a:

a |
5.7 |
14.9 |
0.53 |

b |
6.24 |
4.36 |
3.09 |

a + b |
5.7 + 6.24 = 11.94 |
||

b + a |
6.24 + 5.7 = 11.94 |

*Solution guide:*

a |
5.7 |
14.9 |
0.53 |

b |
6.24 |
4.36 |
3.09 |

a + b |
5.7 + 6.24 = 11.94 |
14.9 + 4.36 = 19.26 |
0.53 + 3.09 = 3.62 |

b + a |
6.24 + 5.7 = 11.94 |
4.36 + 14.9 = 19.26 |
3.09 + 0.53 = 3.62 |

Lesson 2:

Do the addition and then use the commutative property to try again:

a) 9.46+3.8

b) 45.08+24.97

c) 0.07+0.09.

*Solution guide:*

\(a)\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{\,\,9,46}\\

{3,8}

\end{array}} }\\

{13,26}

\end{array}\) retry \(\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{3,8}\\

{9,46}

\end{array}} }\\

{13,26}

\end{array}\)

b) \(\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{45.08}\\

{24.97}

\end{array}} }\\

{70,05}

\end{array}\) retry \(\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{24,97}\\

{45.08}

\end{array}} }\\

{70,05}

\end{array}\)

c) \(\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{0.07}\\

{0.09}

\end{array}} }\\

{\,\,0,16}

\end{array}\) retry \(\begin{array}{*{20}{c}}

{ + \underline {\begin{array}{*{20}{c}}

{0.09}\\

{0.07}

\end{array}} }\\

{\,\,0,16}

\end{array}\)

Lesson 3:

A rectangle is 16.34m wide and 8.32m longer than wide. Calculate the rectangular perimeter that.

*Solution guide:*

The length of the rectangle is:

16.34 + 8.32 = 24.66 (m)

Perimeter of the rectangle is:

(24.66 + 16.34) × 2 = 82(m)

Answer: 82m.

Lesson 4:

One week the shop sold 314.78m2 of fabric, the next week sold 525.22m of fabric. Knowing that the store sells all days of the week, how many meters of fabric on average each day?

*Solution guide:*

The number of meters of fabric the shop sold in two weeks was:

314.78 + 525.22 = 840(m)

The number of days in two weeks is:

7 × 2 = 14 (days)

The average number of meters of fabric sold per day is:

840 : 14 = 60(m)

Answer: 60m of fabric.

.

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