NCERT Class 9 Solutions: Linear Equation in Two Variable (Chapter 4) Exercise 4.3 – Part 6
Q8 In countries like the USA and Canada, temperature is measured in Fahrenheit, whereas in countries like India, it is measured in Celsius. Here is a linear equation that converts Fahrenheit to Celsius.

Draw the graph of the linear equation above using Celsius for and Fahrenheit for

If the temperature is , what is the temperature in Fahrenheit?

If the temperature is , what is the temperature in Celsius?

If the temperature is , what is the temperature in Fahrenheit and if the temperature is , what is the temperature in Celsius?

Is there a temperature which is numerically the same in both Fahrenheit and Celsius? If yes, find it.
Solution:
Given equation is

Draw the graph of the linear equation above using Celsius for and Fahrenheit for
Let temperature in Celsius be and Fahrenheit be
So the equation will be . Note how this equation expresses temperature in Fahrenheit for a given temperature in Celsius.
To graph this equation we find a few points:

Putting the value of in equation (1)
Or ,
So the point is

Putting the value in equation (1)
Or,
So, point is

Putting the value in equation (1)
Or,
So, solution is
Therefore graph becomes


If the temperature is , what is the temperature in Fahrenheit?
If,

If the temperature is , what is the temperature in Celsius?
So,
Or
Or
Or

If the temperature is , what is the temperature in Fahrenheit and if the temperature is , what is the temperature in Celsius?

When ,

When ,
So, required temperature


Is there a temperature which is numerically the same in both Fahrenheit and Celsius? If yes, find it.
Let the same temperature be numerically. This temperature m in Fahrenheit should be same as temperature m in Celsius, i.e. = . We know that . Therefore,
Let’s solve this equation,
So, numerical value of required temperature