Count angles

In the previous lesson, you compared an angle with the corner of a square. Today, we will find the marked angles in a figure and count the ones the question asks for. Mei draws a straight line across a rectangular card. She marks angles a to g. How many of the marked angles are smaller than a right angle? Angles a and b share this corner. Together they make the card's square corner. So each one is smaller than a right angle. Put the square's corner on angle c. Angle c fits inside the corner. So c is smaller than a right angle. Angle d is wider than the square's corner. So d is greater than a right angle. We do not count it. The square's corner fits angles e, f and g exactly. They are right angles. We do not count them either. The angles smaller than a right angle are a, b and c. There are 3 marked angles smaller than a right angle. We have answered the question.

Mei cuts a card into this shape. She marks angles h, j, k, m, n and p inside it. How many of the marked angles are right angles? Try it first. Pause if you like. The square's corner fits h, j, m, n and p exactly. They are right angles. Look at the corner that points in. The square fits the gap outside the card. But angle k is marked inside the card. It is much wider than a square corner. So k is not a right angle. The right angles are h, j, m, n and p. There are 5 marked right angles.

Mei draws three straight lines that cross at one point. She marks angles r, s, t, u, v and w. How many of the marked angles are smaller than a right angle? Try it first. Pause if you like. Put the square's corner here. Angles r and s both fit inside it. So they are smaller than a right angle. Angle t is wider than the corner. Put the square's corner here. Angles u and v both fit inside it. So they are smaller than a right angle. Angle w is wider than the corner. The angles smaller than a right angle are r, s, u and v. There are 4 marked angles smaller than a right angle. Check each marked angle and count it only once. Two or more angles can share the same point.