Question

How do you solve the following system?
$3x + 3y = - 21,{\text{ 4x - y = - 1}}$

Answer 1

Hint: Here we are given two sets of equations and there are two variables in it. So, we will use elimination method to find out the required values for the unknowns.

Complete step by step answer:
Take the given expressions:
$3x + 3y = - 21$
Take common multiple common from the above equation from both the sides of the equation
$x + y = - 7$ …. (A)
${\text{4x - y = - 1}}$ ….. (B)
To use the method of elimination the coefficient of any of the two equations of the same variable should be same.
Multiply equation (A) with using the property of equivalent –
$4x + 4y = - 28$ …. (C)
Subtract equation (B) from the equation (C), where the left hand side of the equation (B) is subtracted from the left hand side of the equation (C) and similarly on the right hand side of the equations.
\[(4x + 4y) - (4x - y) = ( - 28) - ( - 1)\]
When there is a negative sign outside the bracket then the sign of the terms also changes. Positive terms become negative and vice-versa.
\[(4x + 4y - 4x - y = - 28 + 1\]
Make the like terms together.
$\underline {4x - 4x} + \underline {4y - y} = - 28 + 1$
Like terms with equal values and opposite signs cancels each other. Also, when you subtract a bigger number from the smaller number you have given a sign of the bigger number to the resultant value.
$ \Rightarrow 3y = - 27$
Term multiplicative on one side if moved to the opposite side then it goes to the denominator.
$ \Rightarrow y = - \dfrac{{27}}{3}$
Common factors from the numerator and the denominator cancel each other.
$ \Rightarrow y = ( - 9)$
Place above value in equation (A)
$x + ( - 9) = - 7$
Simplify the above equation –
$F
  x - 9 = - 7 \\
  x = - 7 + 9 \\
  x = 2 \\
 $
Therefore, the solutions of the set of equations are – $(x,y) = (2, - 9)$

Note: Always remember that when we expand the brackets or open the brackets, sign outside the bracket is most important. If there is a positive sign outside the bracket then the values inside the bracket does not change and if there is a negative sign outside the bracket then all the terms inside the bracket changes. Positive terms change to negative and negative term changes to positive.