what is the meaning of

  1. A plane surface is one that lies evenly with the straight lines on itself.
  2. A plane angle is the inclination to each other of two lines in a plane, which meet each other and do not lie in a straight line.

Answer :

1 )
 A plane surface is one that lies evenly with the straight lines on itself.
This is the 7th definition of the Euclid's definition .
That states that as we know from the 4th definition  " ​A straight line is a line which lies evenly with the points on itself. " that means line only have one dimension . So from " ​A plane surface is one that lies evenly with the straight lines on itself. " that plane is 2-dimenstion , As it lies between a straight line amd itself ( 2- dimension ) .

2 ) ​A plane angle is the inclination to each other of two lines in a plane, which meet each other and do not lie in a straight line.
That is 8th ​definition of the Euclid's definition .
that states that if two line in a plane and form a angle than they are not a straight to each other . And angle form by can be called as plane angle .
 

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