● Let us now see how this method works for any pair of linear equations in two variables of the form
`a_1 x + b_1 y + c_1 = 0` .........(1)
and `a_2 x + b_2 y + c_2 = 0`.......... (2)
● To obtain the values of x and y as shown above, we follow the following steps:
Step 1 : Multiply Equation (1) by `b_2` and Equation (2) by `b_1` , to get
`b_2 a_1 x + b_2 b_1 y + b_2 c_ 1 = 0`............ (3)
`b_1 a_2 x + b_1 b_2 y + b_1 c_2 = 0` ...........(4)
Step 2 : Subtracting Equation (4) from (3), we get:
`(b_2 a_1 – b_1 a_2) x + ( b_2 b_1 – b_1 b_2) y + (b_2 c_1– b_1 c_2) = 0`
i.e., `(b_2 a_1 – b_1 a_2) x = b_1 c_2 – b_2 c_1`
So, `x= (b_1 c_2 - b_2 c_1 )/( a_1 b_2 - a_2 b_1)` , provided `a_1 b_2– a_2 b_1≠ 0`........ (5)
Step 3 : Substituting this value of x in (1) or (2), we get
` y = (c_1 a_2 - c_2 a_1)/( a_1 b_2 - a_2 b_1 )` ........(6)
● Now, two cases arise :
Case 1 : `a_1 b_2 – a_2 b_1 ≠ 0`. In this case `a_1/a_2 ≠ b_1/b_2` . Then the pair of linear equations has a unique solution.
Case 2 :` a_1 b_2 – a_2 b_1 = 0`. If we write `a_1/a_2 = b_1/b_2 = k` , then `a_1 = k a_2 , b_1 = k b_2` .
Substituting the values of a1 and b1 in the Equation (1), we get
`k (a_2 x + b_2 y) + c_1 = 0`........... (7)
It can be observed that the Equations (7) and (2) can both be satisfied only if
`c_1 = k c_2` , i.e., `c_1/c_2 = k` .
● If `c_1 = k c_2`, any solution of Equation (2) will satisfy the Equation (1), and vice versa. So, if `a_1/a_2 = b_1/b_2 = c_1/c_2 = k` , then there are infinitely many solutions to the pair of linear equations given by (1) and (2).
● If `c_1 ≠ k c_2` , then any solution of Equation (1) will not satisfy Equation (2) and vice versa. Therefore the pair has no solution.
We can summarise the discussion above for the pair of linear equations given by (1) and (2) as follows:
(i) When `color{orange}{a_1/a_2 ≠ b_1/b_2}` , we get a `"unique"` solution.
(ii) When ` color{orange}{a_1/a_2 = b_1/b_2 = c_1/c_2}` , there are `"infinitely"` many solutions.
(iii) When `color{orange}{a_1/a_2 = b_1/b_2 ≠ c_1/c_2}` , there is `"no solution."`
● Note that you can write the solution given by Equations (5) and (6) in the following form :
`x/(b_1 c_2 - b_2 c_1) = y/(c_1 a_2 - c_2 a_1) = 1/(a_1 b_2 - a_2 b_1)`.........(8)
● In remembering the above result, the following diagram may be helpful to you :
● The arrows between the two numbers indicate that they are to be multiplied and the second product is to be subtracted from the first. For solving a pair of linear equations by this method, we will follow the following steps :
Step 1 : Write the given equations in the form (1) and (2).
Step 2 : Taking the help of the diagram above, write Equations as given in (8).
Step 3 : Find x and y, provided `a_1 b_2 – a_2 b_1 ≠ 0`
Step 2 above gives you an indication of why this method is called the
cross-multiplication method.
● Let us now see how this method works for any pair of linear equations in two variables of the form
`a_1 x + b_1 y + c_1 = 0` .........(1)
and `a_2 x + b_2 y + c_2 = 0`.......... (2)
● To obtain the values of x and y as shown above, we follow the following steps:
Step 1 : Multiply Equation (1) by `b_2` and Equation (2) by `b_1` , to get
`b_2 a_1 x + b_2 b_1 y + b_2 c_ 1 = 0`............ (3)
`b_1 a_2 x + b_1 b_2 y + b_1 c_2 = 0` ...........(4)
Step 2 : Subtracting Equation (4) from (3), we get:
`(b_2 a_1 – b_1 a_2) x + ( b_2 b_1 – b_1 b_2) y + (b_2 c_1– b_1 c_2) = 0`
i.e., `(b_2 a_1 – b_1 a_2) x = b_1 c_2 – b_2 c_1`
So, `x= (b_1 c_2 - b_2 c_1 )/( a_1 b_2 - a_2 b_1)` , provided `a_1 b_2– a_2 b_1≠ 0`........ (5)
Step 3 : Substituting this value of x in (1) or (2), we get
` y = (c_1 a_2 - c_2 a_1)/( a_1 b_2 - a_2 b_1 )` ........(6)
● Now, two cases arise :
Case 1 : `a_1 b_2 – a_2 b_1 ≠ 0`. In this case `a_1/a_2 ≠ b_1/b_2` . Then the pair of linear equations has a unique solution.
Case 2 :` a_1 b_2 – a_2 b_1 = 0`. If we write `a_1/a_2 = b_1/b_2 = k` , then `a_1 = k a_2 , b_1 = k b_2` .
Substituting the values of a1 and b1 in the Equation (1), we get
`k (a_2 x + b_2 y) + c_1 = 0`........... (7)
It can be observed that the Equations (7) and (2) can both be satisfied only if
`c_1 = k c_2` , i.e., `c_1/c_2 = k` .
● If `c_1 = k c_2`, any solution of Equation (2) will satisfy the Equation (1), and vice versa. So, if `a_1/a_2 = b_1/b_2 = c_1/c_2 = k` , then there are infinitely many solutions to the pair of linear equations given by (1) and (2).
● If `c_1 ≠ k c_2` , then any solution of Equation (1) will not satisfy Equation (2) and vice versa. Therefore the pair has no solution.
We can summarise the discussion above for the pair of linear equations given by (1) and (2) as follows:
(i) When `color{orange}{a_1/a_2 ≠ b_1/b_2}` , we get a `"unique"` solution.
(ii) When ` color{orange}{a_1/a_2 = b_1/b_2 = c_1/c_2}` , there are `"infinitely"` many solutions.
(iii) When `color{orange}{a_1/a_2 = b_1/b_2 ≠ c_1/c_2}` , there is `"no solution."`
● Note that you can write the solution given by Equations (5) and (6) in the following form :
`x/(b_1 c_2 - b_2 c_1) = y/(c_1 a_2 - c_2 a_1) = 1/(a_1 b_2 - a_2 b_1)`.........(8)
● In remembering the above result, the following diagram may be helpful to you :
● The arrows between the two numbers indicate that they are to be multiplied and the second product is to be subtracted from the first. For solving a pair of linear equations by this method, we will follow the following steps :
Step 1 : Write the given equations in the form (1) and (2).
Step 2 : Taking the help of the diagram above, write Equations as given in (8).
Step 3 : Find x and y, provided `a_1 b_2 – a_2 b_1 ≠ 0`
Step 2 above gives you an indication of why this method is called the
cross-multiplication method.