Second grade math concepts

Doubles
In doubles, both addends are the same.

1 + 1 = 2                2 + 2 = 4
3 + 3 = 6                4 + 4 = 8
5 + 5 = 10             6 + 6 = 12

How to decompose a number
To decompose a number is to break down the number into its two or more parts.

Equal groups
Equal groups are groups that have the same number of items.

Estimate:
No need to get an exact answer. For example, estimate the numbers of hearts you see below. No need to count!
Possible answer: 35, 40 or 30

♥ ♥ ♥ ♥ ♥ ♥ ♥ ♥  
  ♥ ♥ ♥ ♥ ♥ ♥ ♥ ♥    
    ♥ ♥ ♥ ♥ ♥ ♥       
♥ ♥ ♥ ♥ ♥ ♥ ♥ ♥ ♥ ♥ ♥

What is the meaning of a.m. ?

12:00 a.m. : midnight; sleeping time
7:30 a.m. : half past 7; wake up time
10:15 a.m.: a quarter after 10; doing work in class.

Example of angle.
When two lines meet at a common point, they make an angle.

Bar graph
A graph that uses rectangles to compare data. Saturday got 15 votes.

Array
The array you see below is made with triangles. The array has 4 rows and 5 columns.  
4 × 5 = 20 triangles.

▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲

What is the associative property?
No matter how you group the three numbers below before adding, you will still get the same answer.

8 + 5 + 2     =    (8 + 5)  + 2
                           =    8 + (5 + 2)         

Even number
There are 10 balls and 10 is an even number. An even number has 2 equal parts. 10 has 2 equal parts. Each part is equal to 5.

Count back

7 - 4

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☺☺☺☺☺☺
☺☺☺☺☺
☺☺☺☺
☺☺☺

1 Dollar
1 dollar is a currency used in the USA that is equal to 100 cents.
1 dollar  = 100 cents

What is a number line?

Word form of number

The word form of 345 is "three hundred, forty-five"
The word form of 268 is "two hundred sixty-eight"

Odd number
There are 9 balls and 9 is an odd number. An odd number does not have 2 equal parts. One part is is 5 and the other is 4. 

Quadrilaterals
Quadrilaterals are closed figures with 4 straight sides. For example, squares, rectangles, and parallelograms are quadrilaterals.

Standard form
A familiar way of writing numbers using digits.

432

Tens
1 long is a physical representation of 10.

Vertex
A point where 2 or more sides of shape meet. The vertex of the cube you see below is shown with a small white circle.
The vertex of the triangle you see below is shown with a small red circle.

Fourths
The parts that we get when something is divided into 4 equal parts.

What is the meaning of p.m.?

12:00 p.m. : noon ; lunch time
3:30 p.m. : half past 3 ; nap time
6:45 p.m.: a quarter to 7 ; dinner time

Line plot
A line plot shows data on a number line using x or some other marks to show how many times a number appears.
The number 6 appears 3 times.
The number 4 appears once.

What is an equation?
An equation shows that two things are equal. For example the equation 4 + 8 = 12 is saying that 4 + 8 is equal to 12. An equal sign must be there to call something an equation!

4 + 8  =  12
   
14  =   6 + 8

Compare numbers

4 is smaller than 8 or 4 < 8

8 is bigger than 4 or 8 > 4

Centimeter ruler

Centimeter or cm
The width of a finger or the diameter of a wedding ring is about 1 centimeter long.
The length of a baseball bat is about 1 meter.
1 meter = 100 centimeters

Count on
5 + 3

☺☺☺☺☺   
☺☺☺☺☺☺
    ☺☺☺☺☺☺☺
             ☺☺☺☺☺☺☺☺     

USA money
One dollar + 1 quarter + 1 dime + 1 nickel + 1 penny = 1 dollars and 41 cents.

Fractions

Regroup
Regroup 15 ones as 1 ten and 5 ones

Pictograph
Key of a pictograph. Each face = 1 player.
2 people loves soccer.

Flat
10 tens or 10 long  = 1 hundred or 1 flat

Sequence

      +5      +5           +5
      3         8      13     18

Place value
The value of a digit based on where the digit is located. The place value of 4 in 546 is 40.