The blog postings are about the Singapore Math. The readers can learn from the postings on Solving Singapore Primary School Mathematics. The blog presenting the Math Concept, Math Questions with solutions that teaches in Singapore Primary Schools. You or the kids can learn the skills to deal with Math Modeling, Math Problem Solving and Problem Sum from Lower Primary School to Upper Primary School level after reading the blog postings. This posting is an upper primary school math question on Angle and Shape.

Read the posting on **Sarin learns Lines and Angles (Math concept)** to understand the concept of lines and angles. Read also the posting on **Sarin learns Shapes and Solids in school (Math Concept)** to know about shapes.

**Upper primary school mathematics question UPQ308**

The figure below, not drawn to scale, is a semicircle with O as the centre. Find ∠ a.

*Solution:*

A semicircle and O is the centre

AO, CO and BO are radius of the semicircle

AO = BO = CO

AOB formed an isosceles triangle

∠ OAB = ∠ OBA

AOC formed an isosceles tirnagle

∠ OAC = ∠ OCA

CAB fromed a triangle in the semicircle

∠ OAB + ∠ OBA + ∠ OAC + ∠ OCA = 180^{0}

2 ∠ OAB + 2 ∠ OAC = 180^{0}

∠ OAB + ∠ OAC = 180^{0} ÷ 2 = 90^{0}

∠ a = 90^{0}