(i) For which values of a and b does the following pair of linear equations have an infinite number of solutions?
Question.
(i) For which values of a and b does the following pair of linear equations have an infinite number of solutions? 2x + 3y = 7 (a – b) x + (a + b) y = 3 a + b – 2
(ii) For which value of k will the following pair of linear equations have no solution? 3x + y = 1 (2k – 1) x + (k –1) y = 2k + 1.
(i) For which values of a and b does the following pair of linear equations have an infinite number of solutions? 2x + 3y = 7 (a – b) x + (a + b) y = 3 a + b – 2
(ii) For which value of k will the following pair of linear equations have no solution? 3x + y = 1 (2k – 1) x + (k –1) y = 2k + 1.
Solution:
(i) 2x + 3y – 7 = 0 ...(i)
(a – b) x + (a + b) y – (3 a + b – 2) = 0 ...(ii)
For infinite number of solutions, we have
$\frac{\mathbf{a}-\mathbf{b}}{\mathbf{2}}=\frac{\mathbf{a}+\mathbf{b}}{\mathbf{3}}=\frac{\mathbf{3 a}+\mathbf{b}-\mathbf{2}}{\mathbf{7}}$
For first and second, we have
$\frac{\mathbf{a}-\mathbf{b}}{\mathbf{2}}=\frac{\mathbf{a}+\mathbf{b}}{\mathbf{3}}$ or $3 \mathbf{a}-3 \mathbf{b}=2 \mathbf{a}+2 \mathrm{~b}$
or a = 5b ...(i)
From second and third, we have
$\frac{\mathbf{a}+\mathbf{b}}{\mathbf{3}}=\frac{\mathbf{3 a}+\mathbf{b}-\mathbf{2}}{\mathbf{7}}$
or 7a + 7b = 9a + 3b – 6 or 4b = 2a – 6
or 2b = a – 3 ...(ii)
From (i) and (ii), eliminating a,
2b = 5b – 3
b = 1
Substituting b = 1 in (i), we get a = 5
(ii) 3x + y = 1
(2k – 1)x + (k – 1)y = 2k + 1
For no, solution, $\frac{\mathbf{a}_{\mathbf{1}}}{\mathbf{a}_{\mathbf{2}}}=\frac{\mathbf{b}_{\mathbf{1}}}{\mathbf{b}_{\mathbf{2}}} \neq \frac{\mathbf{c}_{\mathbf{1}}}{\mathbf{c}_{\mathbf{2}}}$
$\frac{\mathbf{3}}{2 \mathbf{k}-\mathbf{1}}=\frac{1}{\mathbf{k}-\mathbf{1}} \neq \frac{1}{2 \mathbf{k}+1}$
So, $\frac{\mathbf{3}}{\mathbf{2 k - 1}}=\frac{\mathbf{1}}{\mathbf{k}-\mathbf{1}} \quad \& \frac{\mathbf{1}}{\mathbf{k}-\mathbf{1}} \neq \frac{\mathbf{1}}{\mathbf{2 k}+\mathbf{1}}$
3(k – 1) = 2k – 1 2k + 1 = k – 1
k = 2 k=–2
(i) 2x + 3y – 7 = 0 ...(i)
(a – b) x + (a + b) y – (3 a + b – 2) = 0 ...(ii)
For infinite number of solutions, we have
$\frac{\mathbf{a}-\mathbf{b}}{\mathbf{2}}=\frac{\mathbf{a}+\mathbf{b}}{\mathbf{3}}=\frac{\mathbf{3 a}+\mathbf{b}-\mathbf{2}}{\mathbf{7}}$
For first and second, we have
$\frac{\mathbf{a}-\mathbf{b}}{\mathbf{2}}=\frac{\mathbf{a}+\mathbf{b}}{\mathbf{3}}$ or $3 \mathbf{a}-3 \mathbf{b}=2 \mathbf{a}+2 \mathrm{~b}$
or a = 5b ...(i)
From second and third, we have
$\frac{\mathbf{a}+\mathbf{b}}{\mathbf{3}}=\frac{\mathbf{3 a}+\mathbf{b}-\mathbf{2}}{\mathbf{7}}$
or 7a + 7b = 9a + 3b – 6 or 4b = 2a – 6
or 2b = a – 3 ...(ii)
From (i) and (ii), eliminating a,
2b = 5b – 3
b = 1
Substituting b = 1 in (i), we get a = 5
(ii) 3x + y = 1
(2k – 1)x + (k – 1)y = 2k + 1
For no, solution, $\frac{\mathbf{a}_{\mathbf{1}}}{\mathbf{a}_{\mathbf{2}}}=\frac{\mathbf{b}_{\mathbf{1}}}{\mathbf{b}_{\mathbf{2}}} \neq \frac{\mathbf{c}_{\mathbf{1}}}{\mathbf{c}_{\mathbf{2}}}$
$\frac{\mathbf{3}}{2 \mathbf{k}-\mathbf{1}}=\frac{1}{\mathbf{k}-\mathbf{1}} \neq \frac{1}{2 \mathbf{k}+1}$
So, $\frac{\mathbf{3}}{\mathbf{2 k - 1}}=\frac{\mathbf{1}}{\mathbf{k}-\mathbf{1}} \quad \& \frac{\mathbf{1}}{\mathbf{k}-\mathbf{1}} \neq \frac{\mathbf{1}}{\mathbf{2 k}+\mathbf{1}}$
3(k – 1) = 2k – 1 2k + 1 = k – 1
k = 2 k=–2
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