If α=a,β=b,γ=c and αaabβbccγ=0, then α−aa+β−bb+γ−cγ is equal to :
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Solution
Key Concepts and Formulas:
This problem hinges on the strategic application of determinant properties, particularly:
Determinant of a Matrix: The determinant of a 3×3 matrix a11a21a31a12a22a32a13a23a33 can be expanded as a11(a22a33−a23a32)−a12(a21a33−a23a31)+a13(a21a32−a22a31).
Row/Column Operations: Multiplying all elements of a row or column by a scalar k multiplies the determinant by k. Conversely, a common factor from any single row or column can be taken out of the determinant.
Algebraic Manipulation: Transforming expressions like aα into terms involving α−aa using identities like X−1X=1+X−11 or X=1+X−11 (if X=aα and X−1=aα−a). More directly, aα=aα−a+a=aα−a+1.
Step-by-step Derivation:
We are given the determinant equation:
αaabβbccγ=0
We are also given that α=a,β=b,γ=c, which ensures that terms like α−a are non-zero. For the purpose of dividing by a,b,c, we assume a,b,c=0. If any of a,b,c were zero, the final result holds true (as shown in thought process).
Step 1: Divide columns by a,b,c to introduce ratios.
To simplify the determinant and introduce ratios like α/a, β/b, γ/c, we perform the following operations:
Multiply C1 by 1/a, C2 by 1/b, and C3 by 1/c. To maintain the value of the determinant, we must multiply the entire determinant by abc.
abcα/aa/aa/ab/bβ/bb/bc/cc/cγ/c=0
Since a,b,c=0, abc=0. Thus, we can divide by abc:
α/a111β/b111γ/c=0
Step 2: Introduce new variables for clarity.
Let x=α/a, y=β/b, z=γ/c. The determinant becomes:
x111y111z=0
Step 3: Expand the determinant.
We expand the determinant along the first row:
x(yz−1⋅1)−1(1⋅z−1⋅1)+1(1⋅1−y⋅1)=0x(yz−1)−(z−1)+(1−y)=0xyz−x−z+1+1−y=0xyz−x−y−z+2=0
This is a fundamental identity for this type of determinant.
Step 4: Relate the identity to the required expression.
The expression we need to evaluate is S=α−aa+β−bb+γ−cc.
Let's express x,y,z in terms of the terms in S.
Consider the first term: α−aa.
We know x=aα.
Then x−1=aα−1=aα−a.
So, x−11=α−aa.
Similarly, y−11=β−bb and z−11=γ−cc.
Thus, the expression we need to find is S=x−11+y−11+z−11.
From x−1=α−aa1, we have x=1+α−aa1.
Let A=α−aa, B=β−bb, C=γ−cc.
Then x=1+A1, y=1+B1, z=1+C1.
Substitute these into the identity xyz−x−y−z+2=0:
(1+A1)(1+B1)(1+C1)−(1+A1)−(1+B1)−(1+C1)+2=0
Let X′=1/A, Y′=1/B, Z′=1/C.
(X′+1)(Y′+1)(Z′+1)−(X′+1)−(Y′+1)−(Z′+1)+2=0
Expand the first term:
(X′Y′+X′+Y′+1)(Z′+1)=X′Y′Z′+X′Z′+Y′Z′+Z′+X′Y′+X′+Y′+1
Substitute this back into the equation:
(X′Y′Z′+X′Y′+X′Z′+Y′Z′+X′+Y′+Z′+1)−(X′+1)−(Y′+1)−(Z′+1)+2=0X′Y′Z′+X′Y′+X′Z′+Y′Z′+X′+Y′+Z′+1−X′−1−Y′−1−Z′−1+2=0
Combine like terms:
X′Y′Z′+X′Y′+X′Z′+Y′Z′+(X′−X′)+(Y′−Y′)+(Z′−Z′)+(1−1−1−1+2)=0X′Y′Z′+X′Y′+X′Z′+Y′Z′=0
Now, substitute back X′=1/A, Y′=1/B, Z′=1/C:
ABC1+AB1+AC1+BC1=0
Multiply the entire equation by ABC (since A,B,C are defined, their denominators α−a, etc., are non-zero, and a,b,c=0 implies A,B,C=0):
1+C+B+A=0A+B+C=−1
Therefore, α−aa+β−bb+γ−cc=−1.
Note on the Answer:
Our rigorous derivation consistently leads to a value of −1 for the given expression α−aa+β−bb+γ−cc. However, the provided correct answer is (A) 2. This suggests a common type of typo in competitive exams where the required expression might have been slightly different.
If the expression to be evaluated was instead α−aα+β−bβ+γ−cγ, the solution would be:
We know that α−aα=α−a(α−a)+a=1+α−aa.
Similarly, β−bβ=1+β−bb and γ−cγ=1+γ−cc.
Let Salt=α−aα+β−bβ+γ−cγ.
Then, Salt=(1+α−aa)+(1+β−bb)+(1+γ−cc).
Salt=3+(α−aa+β−bb+γ−cc).
Since we found α−aa+β−bb+γ−cc=−1, the value of this alternative expression would be Salt=3+(−1)=2.
This matches the given correct answer (A).
Final Answer for the given question: The calculated value is -1.