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See diagram.

Draw ABC as any triangle.

Now, draw a straight line parallel to BC through A.

Now angle DAG = angle BAC - opposite angles at an intersection of two lines.

Angle DAE = angle ABC as they are angles formed by a line AB intersecting

two parallel lines - included angle.

Angle GAF = angle EAC - opposite angles at intersection of two lines at A

Angle GAF = angle BAC as BC and FE are parallel. AC is intersecting both

lines. These are included angles.

Sum of these angles at A = angles DAG + GAF + EAD = 180 degrees.

At any point the total angle 360° degrees is divided into 180° and 180° by a

straight line.

So sum of angles A+B+C = 180° degrees

Draw ABC as any triangle.

Now, draw a straight line parallel to BC through A.

Now angle DAG = angle BAC - opposite angles at an intersection of two lines.

Angle DAE = angle ABC as they are angles formed by a line AB intersecting

two parallel lines - included angle.

Angle GAF = angle EAC - opposite angles at intersection of two lines at A

Angle GAF = angle BAC as BC and FE are parallel. AC is intersecting both

lines. These are included angles.

Sum of these angles at A = angles DAG + GAF + EAD = 180 degrees.

At any point the total angle 360° degrees is divided into 180° and 180° by a

straight line.

So sum of angles A+B+C = 180° degrees