Right Triangle Altitude Theorem


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Right Triangle Altitude Theorem:

This theorem describes the relationship between altitude drawn on the hypotenuse from vertex of the right angle and the segments into which hypotenuse is divided by altitude.

Consider a right angled triangle,

ABC
which is right angled at
C
.

Right Triangle Altitude Theorem

If two triangles are similar to each other then,

i) Corresponding angles of both the triangles are equal and

ii) Corresponding sides of both the triangles are in proportion to each other.

Thus, two triangles

ΔABC
and
ΔPQR
are similar if

i)

A
=
P
,
B
=
Q
and
C
=
R

ii)

ABPQ
=
BCQR
=
ACPR

Using constructions in

ΔABC
, let
DCA
=
β
and
DCB
=
α
as shown in fig. 2 below:

Right Triangle Altitude Theorem

From fig. 2, in

ΔADC
and
ΔBDC

ADC
=
BDC
(Both measure
90°
)

As

ΔABC
is right angled at
C
:

C
=
α+β
=
90°
—(1)

In

ADC
,

ADC+DCA+CAD
=
180°
(Angle Sum Property)

CAD
=
180°(90°+β)
=
90°β
=
α
(From equation 1)

Similarly in

BDC
,

BDC+DCB+CBD
=
180°
(Angle Sum Property)

CBD
=
180°(90°+α)
=
90°α
=
β
(From equation 1)

Thus by AA axiom of similarity

ADC CDB

Thus in

ΔADC
and
ΔBDC

CDBD
=
ADCD

 hy
=
xh

 h2
=
xy
h
=
xy

Thus, in a right angle triangle the altitude on hypotenuse is equal to the geometric mean of line segments formed by altitude on hypotenuse.

The converse of above theorem is also true which states that any triangle is a right angled triangle, if altitude is equal to the geometric mean of line segments formed by the altitude.

The above theorem can be easily comprehended by visualizing it.

Right Triangle Altitude Theorem

This figure can be represented by splitting

ΔADC
and
ΔBDC
along its altitude h and these dissected portions can be augmented and arranged into two different ways as shown below:

Right Triangle Altitude Theorem

To complete the triangles in fig 4.1 and 4.2, a square of area

h2
units is added in fig 4.1 and a rectangle of area
xy
units is appended to fig. 4.2. As, both figures represent the same triangle therefore the appended areas must be equal i.e., area of square and rectangle must be exactly the same.

 h2
=
xy
h
=
xy
<

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