A
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D | B
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C
Let ABCD be this parallelogram
AB + BC + CD + DA = 50, but AD = BC and AB = DC, Then:
2.AD + 2.AB = 50 and :
AD+AB = 25 . We also know that AB+1 = AD (Given), then:
AD + (AD+1) = 25
2.AD = 24
AD = 12, THEN AB = 12+1=13
Calculate DB the diagonal perpendicular to AD, using Pythagoras:
AD² + DB² = AB²
12² +DB² = 13²
DB² = 13² - 12² = 25
AND DB= 5