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Cross Multiply
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
Choose Your Level
Cross Multiply
What is it?
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
In simple words
- From ab = cd, cross multiply to get a*d = b*c.
- ab = cd
- a*d = b*c
- x=5
Important terms
Cross Multiply: From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
What you should learn
- Define and explain cross multiply.
- Apply cross multiply to a focused FBISE question.
- Check the answer for correct terms, evidence, working, and units.
Examples
- ab = cd
- a*d = b*c
- x=5
CORE UNDERSTANDING
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
EXAM APPLICATION
State the correct term, connect it to the chapter rule or evidence, and use a clear example or calculation.
Recommended for Class 9
Full school-level explanation
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
Learn the idea, then use it:
WHAT — CROSS MULTIPLY
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
HOW — Use the idea
Identify what is given, select this topic's rule or evidence, apply it one step at a time, and check the result.
CORE UNDERSTANDING
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denominators must stay nonzero. Cross multiply is just multiplying both sides by b*d, so it is not a new law, only a shortcut.
Remember
- ab = cd
- a*d = b*c
- x=5
Examples
- ab = cd
- a*d = b*c
- x=5
- CROSS MULTIPLY: From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives
EXAM APPLICATION
State the correct term, connect it to the chapter rule or evidence, and use a clear example or calculation.
Remember
- correct term
- chapter link
- clear application
Examples
- Define first
- Show the rule or evidence
- Check units, labels, and conclusion
Remember
- ab = cd
- a*d = b*c
- x=5
- keep the ideas separate
- ab = cd
- a*d = b*c
- x=5
- correct term
Deeper understanding
Go beyond memorising Cross Multiply. Connect the definition to reasons, relationships, exceptions, and exam wording.
Common mistakes
Exam focus
Core Idea
Key Points
Simple Check
How to Answer
Exam focus
Quick revision
- ab = cd
- a*d = b*c
- x=5
- keep the ideas separate
- Cross Multiply: From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example:
- WHAT — CROSS MULTIPLY: From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example:
Do not merge cross multiply with a neighbouring idea that uses a different definition, rule, or method.
For long answers on Cross Multiply: define clearly → explain the main parts → give one example → state one exam caution.
Exam Focus
Exam Focus
Important points
- ab = cd
- a*d = b*c
- x=5
- keep the ideas separate
Important formulas
- ab = cd
- a*d = b*c
- x=5
Common exam traps
- Do not merge cross multiply with a neighbouring idea that uses a different definition, rule, or method.
- Do not invent a formula, fact, unit, or diagram label that the chapter does not support.
Long question structure
Definition → key parts → example → exam caution
Common Mistakes
Common Mistakes
- Mistake
Do not merge cross multiply with a neighbouring idea that uses a different definition, rule, or method.
- Mistake
Do not invent a formula, fact, unit, or diagram label that the chapter does not support.
- Mistake
Do not merge cross multiply with a neighbouring idea that uses a different definition, rule, or method.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
ab = cd
a*d = b*c
x=5
keep the ideas separate
From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denomi
ab = cd
- Definition
- From ab = cd, cross multiply to get ad = bc. Then expand and solve the linear equation. Example: x+13 = 42 gives 2(x+1)=12, so x+1=6 and x=5. The denomi
- Formula
- ab = cd
- Example
- ab = cd
- Critical warning
- Do not merge cross multiply with a neighbouring idea that uses a different definition, rule, or method.
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Exam Practice
Define
Q1. Define Cross Multiply.
Explain
Q2. Explain: CORE UNDERSTANDING.
Example
Q3. Give one clear example of Cross Multiply and say why it fits.
Mistake
Q4. State one common mistake students make about Cross Multiply.
Exam note
Q5. Write a short exam note on Cross Multiply: definition, two points, one example.
Sources & References
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.