JEE MAIN - Mathematics (2023 - 29th January Evening Shift - No. 21)
Let A be a symmetric matrix such that $$\mathrm{|A|=2}$$ and $$\left[ {\matrix{
2 & 1 \cr
3 & {{3 \over 2}} \cr
} } \right]A = \left[ {\matrix{
1 & 2 \cr
\alpha & \beta \cr
} } \right]$$. If the sum of the diagonal elements of A is $$s$$, then $$\frac{\beta s}{\alpha^2}$$ is equal to __________.
Answer
5
Explanation
$$A = \left( {\matrix{ a & c \cr c & b \cr } } \right)$$
$$|A| = ab - {c^2} = 2$$ ...... (1)
$$\left( {\matrix{ 2 & 1 \cr 3 & {{3 \over 2}} \cr } } \right)\left( {\matrix{ a & c \cr c & b \cr } } \right) = \left( {\matrix{ 1 & 2 \cr \alpha & \beta \cr } } \right)$$
$$2a + c = 1$$ ..... (2)
$$2c + b = 2$$ ..... (3)
$$3a + {3 \over 2}c = \alpha $$ .... (4)
$$3c + {3 \over 2}b = \beta $$ ..... (5)
From (1), (2) and (3)
$$a = {3 \over 4},b = 3,c = - {1 \over 2}$$
$$\Rightarrow$$ Now $$\alpha = {6 \over 4}$$
$$\beta = 3$$
$$s = {{15} \over 4}$$
$${{\beta s} \over {{\alpha ^2}}} = {{3 \times {{15} \over 4}} \over {{{\left( {{6 \over 4}} \right)}^2}}} = {{{{45} \over 4}} \over {{9 \over 4}}} = 5$$
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