Answer :

The formula for finding the sum of the interior angles of any polygon is 
(n-2) x 180 where n is the no. of sides the polygon has.
So the sum of the interior angles of a rectangle would be
(4-2) x 180
2 x 180 = 360 degrees
So the measure of one angle would be 360/4 (because there are four angles in a rectangle) = 90 degrees
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consider a rectangle ABCD
join BD and AC
now,consider the triangle ABD and BAC
AB=AB (common)
AD=BC
AC=BD(diagonals of a rectangle are equal)
therefore,<A=<B (CPCT)....................................................(1)
now consider the triangle ABCD
as it is a parallelogram,the adjacent angles are supplementary
i.e.<A+<B=180.............................................................(2)
But,<A=<B
so, 2<A=180
<A=90
hence,<b=90 too'
llly,we can prove the rest.. :)

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