NIMCET 2022 Mathematics PYQ — Suppose that the temperature at a point (x, y) on a metal plate i… | Mathem Solvex | Mathem Solvex
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NIMCET 2022 — Mathematics PYQ
NIMCET | Mathematics | 2022
Suppose that the temperature at a point (x, y) on a metal plate is T(x, y) = 4x^2 - 4xy + y^2. An ant, walking on the plate, traverses a circle of radius 5 centred at the origin. What is the highest temperature encountered by the ant?
Choose the correct answer:
A.
125
(Correct Answer)
B.
120
C.
0
D.
25
Correct Answer:
125
Explanation
Method 1 — Parametrise the circle (trig).** On the circle x2+y2=25 put x=5cost,y=5sint. Then
<br>T=4x2−4xy+y2=25(4cos2t−4costsint+sin2t).<br>
Use cos2t=21+cos2t,sin2t=21−cos2t,costsint=2sin2t. So
The expression 1.5cos2t−2sin2t has amplitude 1.52+22=2.25+4=2.5. Thus its maximum is +2.5, so the bracketed term max = 2.5+2.5=5. Hence
<br>Tmax=25⋅5=125.<br>
**Method 2 — Quadratic form / eigenvalues. Write T=[xy](4−2amp;−2amp;1)(xy). On x2+y2=25 the maximum of T is 25 times the largest eigenvalue of the matrix. The matrix has trace 5 and determinant 0, so eigenvalues are 25±5=5,0. Largest eigenvalue =5. Thus Tmax=25⋅5=125.
125(option a)
Explanation
Method 1 — Parametrise the circle (trig).** On the circle x2+y2=25 put x=5cost,y=5sint. Then
<br>T=4x2−4xy+y2=25(4cos2t−4costsint+sin2t).<br>
Use cos2t=21+cos2t,sin2t=21−cos2t,costsint=2sin2t. So
The expression 1.5cos2t−2sin2t has amplitude 1.52+22=2.25+4=2.5. Thus its maximum is +2.5, so the bracketed term max = 2.5+2.5=5. Hence
<br>Tmax=25⋅5=125.<br>
**Method 2 — Quadratic form / eigenvalues. Write T=[xy](4−2amp;−2amp;1)(xy). On x2+y2=25 the maximum of T is 25 times the largest eigenvalue of the matrix. The matrix has trace 5 and determinant 0, so eigenvalues are 25±5=5,0. Largest eigenvalue =5. Thus Tmax=25⋅5=125.