JEE MAIN - Mathematics (2022 - 29th July Evening Shift - No. 9)
Let $$m_{1}, m_{2}$$ be the slopes of two adjacent sides of a square of side a such that $$a^{2}+11 a+3\left(m_{1}^{2}+m_{2}^{2}\right)=220$$. If one vertex of the square is $$(10(\cos \alpha-\sin \alpha), 10(\sin \alpha+\cos \alpha))$$, where $$\alpha \in\left(0, \frac{\pi}{2}\right)$$ and the equation of one diagonal is $$(\cos \alpha-\sin \alpha) x+(\sin \alpha+\cos \alpha) y=10$$, then $$72\left(\sin ^{4} \alpha+\cos ^{4} \alpha\right)+a^{2}-3 a+13$$ is equal to :
119
128
145
155
Explanation
One vertex of square is
$(10(\cos \alpha-\sin \alpha), 10(\sin \alpha+\cos \alpha))$
and one of the diagonal is
$(\cos \alpha-\sin \alpha) x+(\sin \alpha+\cos \alpha) y=10$
So the other diagonal can be obtained as
$(\cos \alpha+\sin \alpha) x-(\cos \alpha-\sin \alpha) y=0$
So, the point of intersection of the diagonal will be
(5( $\cos \alpha-\sin \alpha), 5(\cos \alpha+\sin \alpha))$.
Therefore, the vertex opposite to the given vertex is $(0,0)$.
So, the diagonal length $=10 \sqrt{2}$
Side length $(a)=10$
It is given that
$a^{2}+11 a+3\left(m_{1}^{2}+m_{2}^{2}\right)=220$
$m_{1}^{2}+m_{2}^{2}=\frac{220-100-110}{3}=\frac{10}{3}$
and $m_{1} m_{2}=-1$
Slopes of the sides are tan $\alpha$ and $-\cot \alpha$
$\tan ^{2} \alpha=3$ or $\frac{1}{3}$
$72\left(\sin ^{4} \alpha+\cos ^{4} \alpha\right)+a^{2}-3 a+13$
$=72 \cdot \frac{\tan ^{4} \alpha+1}{\left(1+\tan ^{2} \alpha\right)^{2}}+a^{2}-3 a+13=128$
$(10(\cos \alpha-\sin \alpha), 10(\sin \alpha+\cos \alpha))$
and one of the diagonal is
$(\cos \alpha-\sin \alpha) x+(\sin \alpha+\cos \alpha) y=10$
So the other diagonal can be obtained as
$(\cos \alpha+\sin \alpha) x-(\cos \alpha-\sin \alpha) y=0$
So, the point of intersection of the diagonal will be
(5( $\cos \alpha-\sin \alpha), 5(\cos \alpha+\sin \alpha))$.
Therefore, the vertex opposite to the given vertex is $(0,0)$.
So, the diagonal length $=10 \sqrt{2}$
Side length $(a)=10$
It is given that
$a^{2}+11 a+3\left(m_{1}^{2}+m_{2}^{2}\right)=220$
$m_{1}^{2}+m_{2}^{2}=\frac{220-100-110}{3}=\frac{10}{3}$
and $m_{1} m_{2}=-1$
Slopes of the sides are tan $\alpha$ and $-\cot \alpha$
$\tan ^{2} \alpha=3$ or $\frac{1}{3}$
$72\left(\sin ^{4} \alpha+\cos ^{4} \alpha\right)+a^{2}-3 a+13$
$=72 \cdot \frac{\tan ^{4} \alpha+1}{\left(1+\tan ^{2} \alpha\right)^{2}}+a^{2}-3 a+13=128$
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