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SIMULTANEOUS EQUATIONS

CONTENT

SIMULTANEOUS LINEAR EQUATIONS

Methods of solving Simultaneous equation

  1. Elimination method
  2. Substitution method

iii.         Graphical method

 

ELIMINATION METHOD

One of the unknowns with the same coefficient in the two equations is eliminated by subtracting or adding the two equations. Then the answer of the first unknown is substituted into either of the equations to get the second unknown.

 

Example

Solve for x and y in the equations 2x + 5y   = 1  and 3x – 2y   =   30

Solution

To eliminate x multiply equation 1 by 3 and equation 2 by 2

2x + 5y = 1      ……….                           eqn 1 (x (3)

3x – 2y = 30 …………                eqn 2 (x (2)

Resulting into,

6x + 15 y = 3   ……….               eqn 3

6x – 4y = 60   ………..  eqn 4

Subtract eqn 3 from eqn 4

6x – 6x + 15y – (- 4y) = 3 – 60

19y =   -57   3

19        19

y = -3

            Substitute y = – 3 into eqn 1

2x + 5 (-3) = 1

2x = 1 + 15

2x  =16

2         2

x  =  8

\  y =  -3  and  x = 8

 

Evaluation

Using elimination method to solve the simultaneous equations.

  1. 5x – 4y = 38 and x + 3y = 22
  2. 2c-3d= -4 and 4c-3d= -14

 

SUBSTITUTION METHOD

One of the unknowns (preferably the one having 1 has its coefficient) is made the subject of the formula in one of the equations and substituted into the other equation to obtain the value of the first  unknown which is then substituted into either of the equations to get the second unknown.

 

Example: Solve the simultaneous equation 2x + 5y = 1 and 3x – 2y = 30

Solution

2x + 5y   = 1……………. eq 1

3x – 2y   = 30 ………….. eq 2

Make x the subject in eqn 1

2x   =   1 – 5y

2             2

x =   1 – 5y   …………    eqn 3

2

Substitute eq3 into eqn 2

3 (1-5y)   –   2y = 30

2

Multiple through by 2 or find the LCM and cross multiply.

3 – 15y  –  4y  = 30

2

3 – 15y – 4y = 60

3 – 19y = 60

-19y = 60 – 3

-19y  = 57   3

-19       -19

y  =  – 3

Substitute y = -3 into eq 3

x  =1 – 5y

2

x   =   1 – 5 (-3)1 + 15    =   16

2                 2                        2

x =   8

\ x = 8, y = -3

Evaluation

Solve for x and y in the equations

  1. x + 2y = 10 and 4x + 3y = 20
  2. 4x-y=8 and 5x+y=19

 

SIMULTANEOUS EQUATIONS INVOLVING FRACTIONS

Example

  1. Solve the following equations simultaneously

2 –   1 =  3    and         4  +   3    =   16

x      y                             x       y

Solution

 –    1    =    3

x        y

4   +     3     =    16

x          y

Instead of using x and y as the unknown, let the unknown be (1/x) an (1/y).

2(1/x) –  (1/y)  =  3                     …………….         eqn 1

4 (1/x) – 3 (1/y)   =  16   ……………          eqn 2

Using elimination method, multiply equation 1 by 2 to eliminate x.

See also  VOLUME AND CAPACITY

4(1/x) – 2(1/y)  =   6      ……………..     eqn 3

4 (1/x) + 3(1/y) =   16    …………….         eqn 4

-5 (1/y)   =   -10

-5                  -5

1   =    2

y

\   y = ½

Substitute (1/y) = 2   into eqn 1

2 (1/x) – (1/y) = 3

2 (1/x) – (2) = 3

2(1/x) = 3 + 2

2 (1/x) =   5

1     =       5

x             2

\ x   = 2/5

\ y = ½,  x  =  2/5

Evaluation

  1. Solve for x and y simultaneously, II. Solve the pair of equations for x and y

x  +y    =    1                                                 respectively.

2      2                                                                  2x-1 – 3y-1 = 4

x   –   y  =   1½                                                    4x-1 + y-1 = 1

2        6

 

FURTHER EXAMPLES

Solve for x and y simultaneously: 2x – 3y + 2 = x + 2y – 5 = 3x + y.

Solutions

2x – 3y + 2 = x + 2y – 5 = 3x + y

Form two equations out of the question

2x – 3y + 2 = 3x + y

x + 2y – 5 = 3x + y

OR

2x – 3y + 2 = x + 2y – 5 ————- eq 1

x + 2y – 5 = 3x + y       ————– eq 2

Rearrange the equations to put the unknown on one side and the constant at the other side.

2x – 3y – x – 2y = – 5 – 2

2x – x – 3y – 2y = -7

x – 5y = -7 —————- eq 3

From eqn 2

x – 3x + 2y – y – 5

– 2x + y = 5 ————- eq 4

Using substitution method solve eq 3 & 4

x – 5y = -7 —————- eq 3

-2x + y = 5 ————— eq 4

Make y the subject in eq 4.

y = 5 + 2x ————— eq 5

Substitute eqn 5 into eqn 3.

x – 5 (5 + 2x) = -7

x – 25 – 10x = -7

-9x – 25 = -7

-9x = -7 + 25

-9x = 18

x = 18

-9

X = -2

Substitute x = – 2 into eqn 5

y = 5 + 2x

y = 5 + 2(-2)

y = 5 – 4

y = 1

\ x = -2, y = 1

Example

Solve the equations

5x – y/2  = 1                81x  =  27 3x -y

9

Solution

5x – y/2  = 1             ———– eq 1

81x=  273x -y        ———- eq 2

9

From eq 1 (using the law of indices)

5x – y/2  = 5

x – y/2 = 0

2x – y = 0 ———— eq 3

From eq 2.

81x=  273x -y

9

3 4x    =   3 3(3x-y)

3 2

3 4x-2  =  3 3(3x-y)

By comparison

4x – 2 = 9x – 3y

4x – 9x + 3y = 2

– 5x + 3y =2 ——— eq 4

Solve equation 3 and 4 simultaneously

2x – y = 0 ——— eq 3

-5x + 3y =2 ———- eq 4

Using elimination method: multiply equation 3 by 3

6x – 3y = 0     ——– eq 3

-5x + 3y = 2 ———- eq 4

eq 3 + eq 4

x = 2

Substitute x = 2 into eq 3

2x – y = 0

2 (2) – y = 0

4 – y = 0

4 = 0+y

4 = y

\ x = 2, y=4

 

WORD PROBLEMS LEADING TO SIMULTANEOUS EQUATIONS

Examples

1.Seven cups and eight plates cost N1750, eight cups and seven plates cost N1700. Calculate the cost of a cup and a plate

solution

Let   a cup be x and plate be y

7x + 8y = 1750 ————– eq 1

8x + 7y = 1700 ————– eq 2

 

Multiply eq 1 by 8 and eq 2 by 7 to eliminate x (cups).

56x + 64y = 14000 ———- eq 3

56x + 49y = 11900 ———- eq 4

Subtracting eq 4 from eq 3

15y  =  2100

y =   2100

15

Y = 140

Substitute y = 140 into eq 2

8x + 7y = 1700

8x + 7 (140) = 1700

8x + 980 = 1700

8x = 1700 – 980

8x = 720

x = 720

8

x =  90

\ Each cup cost N90 and each plate cost N140

 

  1. Find a two digit number such that two times the tens digit is three less than thrice the unit digit and 4 times the number is 99 greater than the number obtained by reversing the digit.
See also  LOGARITHM OF NUMBERS

Solution

Let the two digit number be  ab, where a is the tens digit and b is the unit digit

From the first statement,

2a  + 3 = 3b

2a – 3b = -3 ………….eq1

From the second statement,

4(10a + b) – 99 = 10b + a

40a + 4b – 99 = 10b + a

40a – a + 4b – 10b = 99

39a – 6b = 99

Dividing through by 3

13a – 2b = 33 ………….eq2

Solving both equations simultaneously,

a = 3 , b = 3

Hence, the two digit number is 33

EVALUATION

1.The  sum  of  two  numbers  is  110  and  their  difference  is  20. Find the two numbers.

2.A pen  a  ruler  cost  #30.If  the  pen  costs  #8  more  than  the  ruler, how  much  does  each  item  cost ?

 

GENERAL EVALUATION AND REVISION   QUESTION

  1. Solve the following simultaneous equation: 3(2x – y) = x + y + 5 &  5(3x  –  2y) = 2 (x –y) + 1
  2. Five years ago, a father was 3 times as old as his son. Now, their combined ages amount to 110years. How old are they?
  3. A doctor and  three  nurses  in  a  hospital  together  earn  #255 000 per  month, while  three  doctors  and  eight  nurses  together  earn  #720  000   per  month. Calculate (a) how much a doctor earns per month.  (b) How much  a  nurse  earns  per  month.
  4. Solve simultaneously,   2x + 2y = 1; 32x+y = 27
  5. Solve: 2x – 2y + 5 = 3x – 4y + 2 = -1

 

WEEKEND ASSIGNMENT

  1. If (x-y) log106 = log10 216 and 2 x+y =32 , calculate the values of x and y
  2. x=1 , y=4 b. x= 4 , y =1       c. x=-4 , y= 1      d. x=4, y= -1
  3. The point of intersection of the lines 3x- 2y =-12 and x + 2y = 4 is …
  4. (5, 0) b. (3, 4) c. (-2, 5)              d. (-2, 3)
  5. Find the value of (x – y), if 2x + 2y =16 and 8x – 2y = 44 a. 2 b. 4    c. 5     d. 6
  6. If 5 (p +2q) =5 and 4 (p+3q) =16, the value of 3(p+q) is …..     a.0   b. -1   c.2    d. 1
  7. Given 4x – 3y = 11 evaluate y2 – 3x

7x – 4y    23                     3                  a.   -2      b. 3       c. -3        d. 2

 

THEORY

  1. Given that 2 1- x/y = 1/32, find x in terms of y, and hence solve the simultaneous equations

2x + 3y – 30 = 0 and        21- x/y  = 1/32 (WAEC)

  1. A number is made up of two digits. The sum of the digits is 11. If the digits are interchanged, the original number is increased by 9. Find the original number. (WAEC).

See also

QUADRATIC EQUATIONS

GEOMETRIC PROGRESSION

ARITHMETIC PROGRESSION (A. P)

PERCENTAGE ERROR

LOGARITHM OF NUMBERS

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