Chapter 2

Rounding, Bounds and Accuracy

What a rounded measurement actually tells you — and why every measured number is a range rather than a value, which changes what you may claim about a calculation.

หลักสูตรแกนกลาง / สสวท.: จำนวนและพีชคณิต (Number and Algebra)หัวข้อนี้ไม่มีบทตรงกันในหลักสูตรไทยระดับนี้ — สสวท. ไม่ได้แยกเรื่องขอบเขตความคลาดเคลื่อนไว้เป็นบท
Common Core (US): N-Q, 7.NS
Cambridge: Lower Secondary Stage 9 — Number: place value, ordering and rounding

2.1 Significant figures

นิยาม

Significant figures

Counted from the first non-zero digit. In $0.00408$ the significant figures are $4$ , $0$ and $8$ — the leading zeros place the number but carry no information.

●
Which zeros count
- Leading zeros never count: $0.0025$ has two significant figures
- Zeros between digits always count: $2005$ has four
- Trailing zeros after a decimal point count: $2.50$ has three

ตัวอย่าง

Round $0.04967$ to two significant figures.

วิธีทำ

1.First significant figure is the $4$ ; the second is the $9$
2.The next digit is $6$ , so round up
3. $49$ becomes $50$ , carried into the same places

ตอบ $0.050$

ข้อควรระวัง

Look at ONE digit past where you are rounding, never further. Rounding $4.649$ to one decimal place gives $4.6$ ; the $9$ does not push the $4$ up to $5$ first. Rounding in two hops is a real and common error.

📖 2 more sections of this chapter are being checked

The whole chapter is written; we read it back section by section before releasing it, because maths nobody has checked should not be on a public page. What is open is complete — nothing here is abridged.

Read it — now practise it

Generate a worksheet on this topic, or sit a timed mock paper.

Build a worksheet Practice arena