Measuring Angles

Task goals

• Read the size of an angle shown on a protractor to the nearest degree, and state that angles are measured in degrees.
• Classify a given or measured angle as acute, right, obtuse, straight, or reflex, using the $90^\circ$, $180^\circ$ and $360^\circ$ boundaries.
• Estimate the size of an angle by comparing it with $90^\circ$ and $180^\circ$ before measuring, then compare the estimate with the actual measure and state the error.
• Start the reading from the zero of whichever protractor scale lines up with one arm, and convert a reading between the inner and outer scales.
• Calculate the reflex angle at a vertex from the marked acute or obtuse angle, using the fact that the two together make a full turn of $360^\circ$.
• Describe the steps for placing and reading a protractor accurately, and use them to construct a given angle, including one larger than $180^\circ$.
• Identify and explain a protractor error, such as reading the wrong scale, mis-aligning the vertex or baseline, or a classification that contradicts the measurement, and describe how to correct it.
• Find an unknown angle at a point by adding or subtracting adjacent measured angles, and apply angle measurement to practical contexts such as roof slopes, square corners, clock hands, and equal slices of a circle.