✔ Angles on a straight line always add up to 180 degrees.
Q2. If two angles form a straight line and one is 50 degrees, what is the other?
✔ Subtract 50 from 180 to find the missing angle on the line.
Q3. In a triangle, two angles are 60 and 70. What is the third?
✔ Subtract the sum of known angles from 180 to find the third angle.
Show transcript
[Slide 1] Angles Around Us
Imagine Ravi looking at the Tampines MRT roof. He sees triangles and straight lines everywhere. We use geometry rules to find hidden angles.
[Slide 2] Key Geometry Rules
Remember that angles on a straight line add up to 180 degrees. Also, the three angles inside any triangle sum to 180 degrees. These rules help us solve composite figures.
[Slide 3] Worked Example at Hawker Centre
Siti visits a hawker centre and sees two beam angles. They form a straight line, so we draw a bar model. The total length represents 180 degrees divided into units.
[Slide 4] Solving Using Bar Model
We have three bar units making 180 degrees, so one unit is 60 degrees. The larger angle has two units, which equals 120 degrees. This model method helps visualise the parts clearly.
[Slide 5] Common Mistakes to Avoid
Students often guess lines are parallel without proof. You must check for specific markings like arrows. Always write down the given numbers before calculating.
[Slide 6] Lesson Recap
To find unknown angles, apply the standard geometry rules carefully. Use visual models to understand relationships between parts. You are now ready for your PSLE exam.