Geometry and measuresGCSE / IGCSE

Angles in a triangle with algebra

Form an equation from the three interior angles.

  • Add all three interior angle expressions and set the total equal to .
  • Include any angle that is given as a number.
  • After solving, substitute back to check the three angles add to .

Example 1

Find the value of .

Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.

Example 2

Find the value of .

Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.

Your turn

Question 1

Find the value of .

Check answer
Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.
Question 2

Find the value of .

Check answer
Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.
Question 3

Find the value of .

Check answer
Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.
Question 4

Find the value of .

Check answer
Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.
Question 5

Find the value of .

Check answer
Add all three interior angles to make 180°.Apply this to both sides.Apply this to both sides.
Add all three interior angles to make 180°.
Apply this to both sides.Apply this to both sides.
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