2TH §8.1 area of triangles

§8.1 Area of triangles
We do exercise 1 together, step by step.
By this we will discover a Formula for Calculating
the area of any Triangle.
A Movie is included, that explains it in a similar way.
Finally you're going to make some exercises from this §.
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Slide 1: Slide
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This lesson contains 28 slides, with interactive quizzes, text slides and 2 videos.

time-iconLesson duration is: 45 min

Items in this lesson

§8.1 Area of triangles
We do exercise 1 together, step by step.
By this we will discover a Formula for Calculating
the area of any Triangle.
A Movie is included, that explains it in a similar way.
Finally you're going to make some exercises from this §.

Slide 1 - Slide

Calculating an area by enclosing:
If you have to calculate the area of a TRIANGLE,
you can first calculate the area of a RECTANGLE 
in which the triangle lies.
In other words: the triangle is enclosed by the rectangle.
More about this in the coming slides!

Slide 2 - Slide

Work out in your head
and key in 
your answer in the next slide.
timer
1:30

Slide 3 - Slide

1a area= ....

Slide 4 - Mind map

SOLUTION:
area KLMN = 9 x 5 = 
area KPN = 3 x 5 : 2 = 
area PLM = 6 x 5 :2 = 
 area KLP =                  -                  -  
45cm2
7.5cm2
15cm2
45cm2
7.5cm2
15cm2
22.5cm2

Slide 5 - Slide

Think about this.
Try to EXPLAIN.
timer
0:20

Slide 6 - Slide

Solution:
Both triangles are half of rectangle 
KRPN, so they have the same size.

Slide 7 - Slide

Think about this.





timer
0:20

Slide 8 - Slide

Solution:
Triangle RLP and triangle LMP
also have the same size.

Slide 9 - Slide

Think this over!
Try to explain.

timer
0:30

Slide 10 - Slide

Solution:
Triangles KPN and LMP together are 
just as big as triangle KLP.
So:
area KLP = 1/2 x area KLMN

Slide 11 - Slide

Think this over!


timer
0:45

Slide 12 - Slide

Solution:
area KLMN = KL x PR
area KLP = 1/2 x area KLMN, so
area KLP = KL x PR : 2

Slide 13 - Slide

After this first task, a Movie follows.
Note: the teacher talks about 'inches'.   An inch  ≈ 2.5 cm !

Slide 14 - Slide

Slide 15 - Video

After the Movie, the THEORY from the book follows.

Slide 16 - Slide

Slide 17 - Slide

Slide 18 - Slide

Question: 
The area of which triangle you cannot calculate 
right away?
ANS


Slide 19 - Slide


A
ABC
B
KLM
C
PQR

Slide 20 - Quiz

TASK:
How to calculate the area of triangle PQR ?
What do we have to do first?

Slide 21 - Slide

TASK:
How to calculate the area of triangle PQR ?
What length do we have to calculate first?
ANS

Slide 22 - Slide

side ...
A
PQ
B
QR
C
RS

Slide 23 - Quiz

First  RS has to be calculated.
Because RS is the HEIGHT for BASE PQ.
RS is perpendicular to PQ.

Slide 24 - Slide

To calculate height RS we have to use .....

Slide 25 - Mind map

Pythagoras' theorem we have to use here! As follows:


Slide 26 - Slide

Slide 27 - Video

Homework
Do §8.1 Exercises 3 until 7
Check your work with the Solutionsbook
Send in your work in GC

Slide 28 - Slide