Lines, angles & triangles
Angles on a line, crossing lines, and the angles of a triangle.
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The three angles in any triangle add up to 180°.
Look: 50° + 60° + 70° = 180°.
✏️ Your turn!
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1Each angle of an equal-sided triangle?
2One angle on a line is 112°. The other?
3Angles 2x, 3x and 4x in a triangle. Find x.
! Careful!
✗A triangle with angles 100° and 90°.
✓That is already 190°. Impossible!
✗Thinking big triangles have more degrees.
✓Every triangle has 180°.
See the whole lesson on one page
The three angles in any triangle add up to 180°.
If you know two angles, take them away from 180° to find the third. Angles on a straight line also add to 180°.
Think of it like thisTear the three corners off a paper triangle. Put them side by side: they always make a straight line, which is 180°.
Solved together
Two angles of a triangle are 35° and 95°. Find the third.
Add the two you know.
35 + 95 = 130Take that from 180°.
180 − 130 = 50
AnswerThe third angle is 50°.
For older learners and grown-ups: a little more
In a right-angled triangle, the longest side c (opposite the right angle) follows Pythagoras: a² + b² = c². With sides 6 and 8: 36 + 64 = 100, so c = 10.
Now practise it
Kanakku makes new questions on lines, angles & triangles every time. It shows the working and gives a hint when you are stuck.