Mathematics
Grade-8
Easy

Question

If C D ‖ B A, find the measure of angles 1, 2, and 3 and name the triangle based on its angles.

  1. angle 3 equals 60 degree text  and  end text increment A B C text  is an equilateral triangle.  end text
  2. angle 3 equals 90 degree text  and  end text increment A B C text  is a right triangle.  end text
  3. angle 3 equals 45 degree text  and  end text increment A B C text  is an isosceles triangle.  end text
  4. angle 3 equals 90 degree text  and  end text increment A B C text  is a right isosceles triangle.  end text

hintHint:

We know alternate interior angles are equal.
The sum of all the interior angles of a triangle is 180.

The correct answer is: angle 3 equals 90 degree text  and  end text increment A B C text  is a right isosceles triangle.  end text


    Since C D ‖ B A comma so, angle 1 and 45 are the alternate interior angles.
    We know alternate interior angles are equal.
    So, angle 1 = 45o
    text  Also, in triangle  end text ABC comma AB equals BC
    Since the angles opposite to equal sides are equal, we get angle 1 equals angle 2
    text  Hence,  end text angle 1 equals angle 2 equals 45 degree
    Now, in triangle ABC,
    The sum of all the interior angles of a triangle is 180o.
    rightwards double arrow angle 1 plus angle 2 plus angle 3 equals 180 degree
    rightwards double arrow 45 degree plus 45 degree plus angle 3 equals 180 degree
rightwards double arrow    angle 3 equals 180 degree minus 90 degree
rightwards double arrow    angle 3 equals 90 degree
    text  So, the measure of  end text angle 3 text  is  end text 90 degree text , so the triangle  end text ABC text  is a right isosceles triangle.  end text

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