Drawing an angle

Vidya wants to draw an angle of 30° and name it ∠TIN. The vertex is I; the arms are IT and IN. She keeps IN as the base arm, and turns the other arm, IT, 30° away from it.

Her four steps:

• Step 1: draw the base arm IN with a ruler.
• Step 2: put the protractor's centre exactly on I, with IN along the 0 line.
• Step 3: starting from the 0 on arm IN, count up that scale to 30 and mark a point T.
• Step 4: join I and T with a ruler.

Now ∠TIN = 30°.

Be careful in Step 3. Use the scale whose 0 lies on the base arm IN. The two scales add up to 180 at every mark, so if you mark 30 on the other scale you will draw 180° − 30° = 150° instead.

The same four steps draw any angle up to 180°. For 110°: draw the base arm, put the centre on its end point with the arm along 0, count up the same scale to 110, mark the point, and join it with a ruler.