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Grade 9 Math, Explained

CDC’s Compulsory Mathematics for Grade 9, unit by unit: every method worked step by step, every exercise question answered, and diagrams to play with.

Start reading: Unit 1 · Sets →

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  1. Unit 1 Sets Nepal has 8 of the world's 14 highest mountains and China has 9, yet that makes only 13. How do you count things that belong to two lists at once? Sets  
  2. Unit 2 Tax Income tax band by band under 2083/84’s new rates, the deductions that lower it, and how 13% VAT is collected a little at each step to the shop. Arithmetic  
  3. Unit 3 Commission and Dividend When a company announces a 40% dividend, 40% of what? And how much of it reaches a shareholder’s bank account? Arithmetic  
  4. Unit 4 Household Arithmetic Use twice the electricity and the bill comes to more than twice as much. How does a meter turn a reading into rupees, and how do you check the bill? Arithmetic  
  5. Unit 5 Area How do you find the area of a plot with three unequal sides, when you can measure its sides but not its height, and what is it in ropani or kattha? Mensuration  
  6. Unit 6 Prism Two tanks can hold the same water yet need different amounts of concrete. Which shape of box holds the most for the least material? Mensuration  
  7. Unit 7 Cylinder and Sphere Why are so many water tanks round, and why does a ball hold the most for the least material of any shape? Mensuration  
  8. Unit 8 Sequence and Series Paris 2024 was the 33rd Olympiad. In which year will the 40th be, and how can you find any term of a list without writing out every one before it? Algebra  
  9. Unit 9 Factorization How much concrete goes into a hollow cubical tank, and why is (a + b)³ so much more than a³ + b³? Algebra  
  10. Unit 10 Highest Common Factor and Lowest Common Multiple Two broods of cicadas, one every 13 years and one every 17, came out together in 2024. When will they meet again? HCF and LCM, now with letters. Algebra  
  11. Unit 11 Linear Equation A cage holds pheasants and rabbits: 35 heads, 94 feet. How many of each? Two facts, two unknowns, and three ways to find the pair that fits both. Algebra  
  12. Unit 12 Indices Fold a sheet of paper 42 times and it would reach the Moon. Counting tells you why, and what 2⁰, 2⁻³ and 8 to the power ⅓ must mean. Algebra  
  13. Unit 13 Triangle How tall is a tree you can’t climb? A mirror, two triangles of the same shape and three short measurements tell you. Geometry  
  14. Unit 14 Quadrilateral Push a frame of four strips and it leans, yet stays a parallelogram. Why, and how does a mason know its corners are square? Geometry  
  15. Unit 15 Construction Four sides measured with a tape don’t fix a field’s shape: how many measures do, and how do a ruler and a compass turn them into an exact drawing? Geometry  
  16. Unit 16 Circle A broken bit of a pot’s rim tells how wide the whole pot was. Why does a chord’s perpendicular bisector always pass through the center? Geometry  
  17. Unit 17 Classification and Presentation of Data Forty marks in a list are a jumble. How do you sort them into a table and draw them so that the pattern jumps out? Statistics and probability  
  18. Unit 18 Measures of Central Tendency Kathmandu’s “average month” gets 125 mm of rain, yet most months get far less. When one number stands for a whole list, which one tells the truth? Statistics and probability  
  19. Unit 19 Probability In Ludo a six can seem to take forever. Is it really rarer than a three? How do we put a number on chance, and do experiments agree with counting? Statistics and probability  
  20. Unit 20 Trigonometry How high up a wall does a ladder reach, from only its length and the angle it leans at? In a right-angled triangle the angle alone fixes the shape. Trigonometry  

Practice

How to use this book

Each unit covers its part of the curriculum in the order of CDC’s textbook, whose page numbers are under each unit’s title; whichever book your school uses, you can read it alongside. Inside you’ll find:

  • Worked examples: try them first; the answer stays hidden until you open it.
  • Try it questions and each unit’s questions at the end, with hints and answers to open when you’re ready.
  • Diagrams to play with: drag, slide and change things to see what happens.
  • Did you know? boxes with the stories and surprises behind the maths.

A companion to your textbook by Science Playground, not published by or connected with CDC, and not a replacement for the textbook: study from the book your school uses, and where the two differ, follow your teacher. Every fact here has been checked against the textbook and other reliable sources, but these pages have not yet been reviewed by subject experts. If you find a mistake, please tell us.