Pro:07

According to the property 7 of the parallelogram (A quadrilateral that has the opposite opposite angles equal is a // gram)
ITAR is a //gram.The sum of the angles of the // gram is 360⁰,

I +

T +

A +

R = 360⁰
2× (

I +

T ) = 360⁰ (because

T =

R and

I =

A )
(

I +

T ) = 180⁰ . thus two opposite angles of ITAR are supplementary..
Let's draw the bisectors of angles

T and

A which intersect in L.
Let's show that

L = 90⁰ ,

T and

A are 2 non-opposing angles of the // gram, they are supplementary angles, so their half-sum is right.
Take the triangle ALT, it's right triangle at L and

T +

A = 90⁰
In the same way we show that the other vertices of the quadrilateral LOUP are right, so it is a rectangle.

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