## Year 2 Lesson Plan 7 - Circles: ,
area, circumference and rounding

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- (5 min) Mental Math
- Find 50% of 700, add 50, now divide by 10 [40]
- Start with 1/2; add 7 and 5/10; now subtract 3 [5]
- Find the decimal equivalent of 3/4; subtract .05; now add .3 [1]
- Find the perimeter of a regular hexagon with 8 cm sides [48 cm]
- Find the area of a 50 by 90 foot rectangle [4500 sq. ft.]
- Is there a remainder when you divide 4,356 by 5? [yes]

- (5 min) Review of selected problems from lesson 6 (no more than 3
problems)

- (5 min) Rounding review

- 4 kids can safely ride in a car. Mrs Latimer's class of 29 kids wants to go on a field trip to the Science center. How many cars will they need? 29/4 = 7.25 cars, but we cannot take .25 car, so we round up to the next car = 8 cars.
- 6 cans of orange juice cost $1.39. I want one can, so I divide $1.39 by 6. My calculator shows .231666666. How much will I have to pay. I do not have .16666 cents, so I will round up to .24 or 24 cents. In this case I rounded up to the nearest hundre
dth (1 cent) of a dollar.

Usually you round up if the digit of interest is 5 or greater, and round
down if the digit is 0-4. However I must use common sense in taking
fractions of cars, and buying fractions of boxes. When you solve a
problem, read the instructions closely to see what form the answer should
be.

- (5 min) Circles and

- The
*diameter* of a circle is a line through its center.
- The
*radius* of a circle is half of the diameter. It is a line from the center out to the edge.
- The
*circumference* of a circle is the distance around it. It is
computed as: d. If r = 5 in. the diameter is 10 in. and the
circumference = 3.14 x 10 = 31.4 in.

- The area of a circle is r
^{2}. If r = 5 in,
the area is 3.14 x 25 or about 78.5 sq. in.

SPECIAL OBJECTIVE: Discuss accuracy and precision. What is the value of
? The ancient
Hebrews thought it was 3. This is too low.

We have been using 3.14 (still too low). The Greeks approximated it by
using 22/7, which is 3.1428 (Too high) If you are a stickler for
precision you use:

3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37410 and so on

The value has been calculated to more than 100,100 places without
repeating, and still it goes on. The Greeks tried long and hard to get an
exact number for .

What shall we use in this class? (Ask for opinions.) WE CAN ALWAYS BE
PRECISE AND JUST SAY . Some quizzes ask for this form.
- (Remainder of class) In-class exercise

- Hand out homework as students successfully complete the in-class exercise.