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# The sum of three numbers is 6. When second number is subtracted from thrice the sum of first and third number, we get number 10. - HSC Science (Computer Science) 12th Board Exam - Mathematics and Statistics

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ConceptElementary Operation (Transformation) of a Matrix

#### Question

The sum of three numbers is 6. When second number is subtracted from thrice the sum of first and third number, we get number 10. Four times the sum of third number is subtracted from five times the sum of first and second number, the result is 3. Using above information, find these three numbers by matrix method.

#### Solution

Given that the sum of three numbers, x, y and z is 6.
From the given statement, we have,
3 (x+z)- y=10
5 (x+y)- 4z= 3
Thus, the system of equations are:
x+y+z=6
3x-y+3z=10
5x+5y-4z=3

Let us write the above equations in the matrix form as:

[[1,1,1],[3,-1,3],[5,5,-4]][[x],[y],[z]]=[[6],[10],[3]]

AX=B

A=[[1,1,1],[3,-1,3],[5,5,-4]],X=[[x],[y],[z]],B=[[6],[10],[3]]

Thus ,[[1,1,1],[3,-1,3],[5,5,-4]]A^-1=[[1,0,0],[0,1,0],[0,0,1]]

Thus ,[[1,1,1],[3,-1,3],[5,5,-4]]A^-1=[[1,0,0],[0,1,0],[0,0,1]]

Thus ,[[1,1,1],[3,-1,3],[5,5,-4]]A^-1=[[1,0,0],[0,1,0],[0,0,1]]

where A=[[1,1,1],[3,-1,3],[5,5,-4]],X=[[x],[y],[z]] and B=[[6],[10],[3]]

Thus, A-1 exists
We know that AA-1 = I

"Thus " ,[[1,1,1],[3,-1,3],[5,5,-4]]A^-1=[[1,0,0],[0,1,0],[0,0,1]]

Applying R2→ R2 - 3R1 , we have

[[1,1,1],[0,-4,0],[5,5,-4]] A^-1=[[1,0,0],[-3,1,0],[0,0,1]]

Applying R3→ R3- 5R1 , we have

[[1,1,1],[0,-4,0],[0,0,-9]] A^-1=[[1,0,0],[-3,1,0],[-5,0,1]]

Applying R3 →R3/-9, we have

[[1,1,1],[0,-4,0],[0,0,1]] A^-1=[[1,0,0],[-3,1,0],[-5/9,0,-1/9]]

Applying R2→R2/4 , we have

[[1,1,1],[0,1,0],[0,0,1]] A^-1=[[1/4,1,0],[3/4,-1/4,0],[5/9,0,-1/9]]

Applying R1→ R1- R2 , we have

[[1,0,1],[0,1,0],[0,0,1]] A^-1=[[1/4,1/4,0],[3/4,-1/4,0],[5/9,0,-1/9]]

Applying R1→R1-R3 , we have

[[1,0,0],[0,1,0],[0,0,1]] A^-1=[[11/36,1/4,1/9],[3/4,-1/4,0],[5/9,0,-1/9]]

AX=B

A-1 AX=A-1 B
IX=A-1 B

X=A-1 B

X=[[11/36,1/4,1/9],[3/4,-1/4,0],[5/9,0,-1/9]][[6],[10],[3]]

[[x],[y],[z]]=[[1],[2],[3]]

Thus, the numbers are 1, 2 and 3.

Is there an error in this question or solution?

#### APPEARS IN

2014-2015 (March) (with solutions)
Question 2.2.3 | 4.00 marks

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Solution The sum of three numbers is 6. When second number is subtracted from thrice the sum of first and third number, we get number 10. Concept: Elementary Operation (Transformation) of a Matrix.
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