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4. AB and CD are respectively the smallest and longest sides of a quadrilateral ABCD (see Fig.). Show that \small \angle A>\angle C and \small \angle B>\angle D.

     

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Consider \Delta ADC in the above figure : 

AD\ <\ CD  (Given)

Thus  \angle CAD\ > \angle ACD (as angle opposite to smaller side is smaller)      

Now consider \Delta ABC,

We have   BC\ > AB

and  \angle BAC\ > \angle ACB

Adding the above result we get,

  \angle BAC\ +\ \angle CAD > \angle ACB\ +\ \angle ACD

 \small \angle A>\angle C

Similarly, consider \Delta ABD,

we have AB\ <\ AD  

Therefore  \angle ABD\ > \angle ADB

and in  \Delta BDC  we have CD\ >\ BC

and  \angle CBD\ >\ \angle CDB

from the above result we have,

 \small \angle B>\angle D

Hence proved.

Posted by

Sanket Gandhi

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