Q1. Ravi buys a triangular ruler for $3.50. If two angles are 40 degrees and 60 degrees, what is the third angle?
✔ The sum of angles in a triangle is 180 degrees. Subtracting 40 and 60 from 180 leaves 80 degrees.
Q2. Siti measures a triangular sign at the MRT. One angle is 90 degrees and another is 45 degrees. Find the missing angle.
✔ Subtract the known angles from the total sum. One hundred eighty minus one hundred thirty-five equals forty-five degrees.
Q3. Wei Ming thinks the angle sum of a triangle is 360 degrees. Is he correct?
✔ The angle sum of any triangle is always 180 degrees. Three hundred sixty degrees is for quadrilaterals.
Show transcript
[Slide 1] Triangles in Our Daily Life
Look at roof tiles or pizza slices around you. Notice the three corners inside every triangle. Imagine folding them to form a straight line.
[Slide 2] The Angle Sum Rule
All triangles have an angle sum of one hundred and eighty degrees. This rule applies to any size of triangle. It helps us find missing values easily.
[Slide 3] Worked Example with Bar Model
Mei Ling buys lunch at Tampines Hawker Centre for eight dollars fifty cents. She notices a triangular sign above the stall. Two angles are fifty degrees and sixty degrees.
[Slide 4] Solving the Missing Angle
Subtract the known part from the whole bar. One hundred eighty minus one hundred ten gives seventy. The third angle is seventy degrees.
[Slide 5] Common Mistake to Avoid
Do not add all three angles together again. Remember the total must be exactly one hundred and eighty. Check your subtraction carefully before writing answers.
[Slide 6] Quick Recap for PSLE
Total angle sum is always one hundred and eighty degrees. Use bar models to visualise parts and whole. Practice with local examples like signs.