Four quantities a,b,c d are siad to be proportion if ______

Explanation

To determine when four quantities \( a, b, c, d \) are said to be in proportion, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Proportion**:
Four quantities \( a, b, c, d \) are said to be in proportion if the ratio of the first quantity to the second is equal to the ratio of the third quantity to the fourth. This can be expressed mathematically as:
\[ \frac{a}{b} = \frac{c}{d} \]

2. **Cross-Multiplication**:
To check if the above condition holds, we can use cross-multiplication. This means we multiply the extremes and the means:
\[ a \cdot d = b \cdot c \]
Here, \( a \) and \( d \) are the extremes, while \( b \) and \( c \) are the means.

3. **Conclusion**:
If the equation \( a \cdot d = b \cdot c \) is true, then we can conclude that the four quantities \( a, b, c, d \) are in proportion.

### Final Answer:
Four quantities \( a, b, c, d \) are said to be in proportion if \( a \cdot d = b \cdot c \).

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