ABCD is a trapezium in which side AB is parallel to the side DC

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50. ABCD is a trapezium in which side AB is parallel to the side DC and E is the mid-point of side AD (see figure). If F is a point on the side BC such that the segment EF is parallel to the side DC, prove that F is the mid-point of BC and EF = 1 half (AB + DC).



Given: ABCD is a trapezium in which side AB is parallel to the side DC and E is the mid-point of side AD. F is a point on the side BC such that the segment EF is parallel to the side DC.

To Prove: F is the mid-point of BC

and         EF equals 1 half left parenthesis AB plus DC right parenthesis


Given: ABCD is a trapezium in which side AB is parallel to the side D

Construction: Join AC to intersect EF at G.
Proof:
(i) In ∆ADC,
∵ E is the mid-point of AD and
EG || DC    | ∵ EF || DC
∴ G is the mid-point of AC
| by converse of mid-point theorem Again, In ∆CAB,
∵ G is the mid-point of AC | proved above
and    GF || DC    | ∵ EF || DC
∴ F is the mid-point of BC | by converse of
mid-point theorem
(ii) We shall prove that the line segment joining the mid-points of any two sides of a triangle is parallel to the third side and half of it.


Given: ABCD is a trapezium in which side AB is parallel to the side D

Let ABC be a triangle in which D and E are the mid-points of the sides AB and AC respectively.
We produce DE to F such that DE = EF and join FC.
In ∆AED and ∆CEF,
AE = CE
| ∵ E is the mid-point of AC
∠AED = ∠CEF
| Vertically opposite angles
ED = EF    | by construction
∴ ∆AED ≅ ∆CEF
| SAS congruence rule
∴ AD = CF    |CPCT
and ∠ADE = ∠CFE    | CPCT
Now, ∵ D is the mid-point of AB
∴ AD = DB
But AD = CF    | Proved above
∴ DB = CF    ...(1)
Again, ∵ DF intersects AD and FC such that
∠ADE = ∠CFE    | Proved above
But these angles form a pair of equal alternate interior angles
∴ AD || FC
⇒ DB || FC    ...(2)
In view of (1) and (2),
DBCF is a parallelogram
| A quadrilateral is a parallelogram if one pair of its opposite sides is parallel and equal
∴ DF = BC
| Opposite sides of a parallelogram are equal
⇒ 2DE = BC    | by construction

rightwards double arrow space space space DE equals 1 half BC

Also, ∵ DBCF is a parallelogram
∴ DF || BC
| Opposite sides of a parallelogram are parallel
⇒ DE || BC
(iii) Using this result in ∆ADC,

EG equals 1 half DC space space space space space space space space space space space space space space space space space space space space space space space space space.... left parenthesis 3 right parenthesis

| ∵ E is the mid-point of AD and G is the mid-point of AC
and in ∆CAB,

GF equals 1 half AB space space space space space space space space space space space space space space space space space space space space space... left parenthesis 4 right parenthesis

| ∵ G is the mid-point of AC and F is the mid-point of BC
Adding (3) and (4), we get

EG plus GF equals 1 half DC plus 1 half AB
rightwards double arrow space space space EF equals 1 half left parenthesis AB plus DC right parenthesis

Also, EG || DC
⇒ EF || DC

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