Question

If ABCD is a quadrilateral whose diagonals AC and BD intersect at O prove that (AB+BC+CD+DA) is greater than 2(AC+BD)

Answer 1

ABCD is a quadrilateral and AC, and BD are the diagonals.

Sum of the two sides of a triangle is greater than the third side.

So, considering the triangle ABC, BCD, CAD and BAD, we get
AB + BC > AC ..........(1)
CD + AD > AC .........(2)
AB + AD > BD .........(3)
BC + CD > BD .........(4)

Adding all the above equations,

2(AB + BC + CA + AD) > 2(AC + BD)
⇒ 2(AB + BC + CA + AD) > 2(AC + BD)
⇒ (AB + BC + CA + AD) > (AC + BD)
⇒ (AC + BD) < (AB + BC + CA + AD)

Answer 2

ABCD is a quadrilateral and AC, and BD are the diagonals. Sum of the two sides of a triangle is greater than the third side. So, considering the triangle ABC, BCD, CAD and BAD, we get AB + BC > AC CD + AD > AC AB + AD > BD BC + CD > BD Adding all the above equations, 2(AB + BC + CA + AD) > 2(AC + BD) ⇒ 2(AB + BC + CA + AD) > 2(AC + BD) ⇒ (AB + BC + CA + AD) > (AC + BD) ⇒ (AC + BD) < (AB + BC + CA + AD)