Download Area review

Area review
 Area is the space inside a 2-D shape.
 Area is always in square units (for example, in2, ft2, yds2, cm2, m2).
Think of covering a shape with a bunch of square area rugs.
 Be careful not to confuse area with perimeter!
 Story problem strategy: Draw a diagram of the situation.
Finding area
Rectangle
w
(working “forwards”)
Given area, finding a
side length [harder]
Multiply length by width.
(working “backwards”)
Divide area by the width to
find the length.
A = Lw
L=
L
Parallelogram
Multiply base by height.
h
h=
NOTE: Height is always at a
right angle with the base. It’s
never the slanting side.
A = s2
s
HINT: You use squaring for
squares, not for rectangles or
parallelograms.
𝐿
𝐴
𝑏
Divide area by the height to
find the base.
b=
Square the side length.
𝐴
Divide area by the base to
find the height.
A = bh
Square
𝑤
Divide area by the length to
find the width.
w=
b
𝐴
𝐴
ℎ
Take the square root of the
area to find the side length.
s = √𝐴
HINT: You use square roots
for squares, not for rectangles
or parallelograms.
D. Stark 4/2/2016
Finding area
Multiply base by height and
then find half of that (either
multiply by ½ or divide by 2).
A triangle is half a
parallelogram.
Triangle
h
b
A = ½ bh
or
h
A=
b
h
b
EXAMPLE (requires algebra):
 If the area of a triangle
is 60 m2, and the base
is 20, what’s the
height?
A = ½ bh
60 = ½ (20)h
60 = 10h
𝑏ℎ
60
2
10
NOTE: Height is always at a
right angle with the base. It’s
never the slanting side.
=
10ℎ
10
h=6m
Find the average of the two
bases. Then multiply by
height.
Trapezoid
b2
h
b1
Given area, finding a
side length [harder]
A = ½ (b1 + b2)h
NOTE: Height is always at a
right angle with the base. It’s
never the slanting side.
NOTE: It doesn’t matter
which you call b1 and which
b2.
Circle
r
Multiply  by the radius
squared. Use 3.14 as an
approximation for .
A =  r2
d
NOTE: Don’t confuse area
with circumference
(C= 2r or C= d
EXAMPLE (requires algebra):
 If the area of a circle is
78.5 m2, what’s the
radius?
A =  r2
78.5  3.14r2
78.5
3.14

25 = r2
r=5m
3.14𝛤2
3.14
[To find r, take the
square root.]
D. Stark 4/2/2016