Let y = y(x) be the solution of the differential equation xsin ( y x )dy = (ysin ( y x ) - x )dx, y(1) = π 2 and let α = cos ( y(e^12) e^12 ). Then…
JEE Main 2026 — Mathematics Calculus
2026integermedium
Let y=y(x) be the solution of the differential equation xsin(xy)dy=(ysin(xy)−x)dx, y(1)=2π and let α=cos(e12y(e12)). Then the number of integral values of p, for which the equation x2+y2−2px+2py+α+2=0 represents a circle of radius r≤6, is __________.
Official previous-year question
Held on 5 Apr 2026 · Verified 6 Jul 2026.
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Solution
Given the differential equation:
xsin(xy)dy=(ysin(xy)−x)dx
Rearranging the terms, we get:
xsin(xy)dy−ysin(xy)dx=−xdx
Dividing both sides by x2:
sin(xy)(x2xdy−ydx)=−xdx
sin(xy)d(xy)=−xdx
Integrating both sides:
−cos(xy)=−ln∣x∣−C
cos(xy)=ln∣x∣+C
Using the initial condition y(1)=2π:
cos(2π)=ln(1)+C⇒C=0
Thus, the solution to the differential equation is:
cos(xy)=ln∣x∣
We need to find α=cos(e12y(e12)). Substituting x=e12:
α=ln(e12)=12
The given equation of the circle is:
x2+y2−2px+2py+α+2=0
Substituting α=12:
x2+y2−2px+2py+14=0
The radius r of the circle is given by:
r=(−p)2+p2−14=2p2−14
For the equation to represent a real circle, r>0:
2p2−14>0⇒p2>7
We are given that r≤6:
2p2−14≤6
2p2−14≤36
2p2≤50⇒p2≤25
Combining the inequalities:
7<p2≤25
Since p is an integer, the possible values for p2 are 9,16,25.
This gives p∈{−5,−4,−3,3,4,5}.
Therefore, there are 6 integral values of p.
Answer: 6
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