MathematicsLinear Differential EquationsJEE Advanced 2009Difficult
Visualized Solution (English)
Part A: Analyzing xesinx−cosx=0
- Let f(x)=xesinx−cosx for x∈(0,2π)
- Check boundaries: f(0)=0⋅e0−cos0=−1<0
- f(2π)=2πesin(π/2)−cos(2π)=2πe>0
- By Intermediate Value Theorem, at least one root exists in (0,2π).
Monotonicity of f(x)
- Differentiate: f′(x)=dxd(xesinx)−dxd(cosx)
- f′(x)=esinx+xesinxcosx+sinx
- For x∈(0,2π), sinx>0, cosx>0, and esinx>0
- Thus, f′(x)>0⟹f(x) is strictly increasing.
- Conclusion: Exactly one solution exists. (Matches 1)
Part B: Intersection of Planes
- For planes to intersect in a line, the system must have infinite solutions.
- Condition: Δ=k424k2121=0
- Expand along the first row: k(k−4)−4(4−4)+1(8−2k)=0
Solving for k in Part B
- Simplify: k2−4k+8−2k=0
- k2−6k+8=0
- Factorize: (k−2)(k−4)=0
- Values of k: k=2 or k=4. (Matches 2, 4)
Part C: Absolute Value Equation
- Let f(x)=∣x−1∣+∣x−2∣+∣x+1∣+∣x+2∣
- Minimum value occurs for x∈[−1,1]: f(0)=1+2+1+2=6
- Equation: f(x)=4k. For solutions to exist, 4k≥6.
Checking Integer Solutions for k
- If k=1,4k=4<6 (No solution)
- If k=2,4k=8. f(2)=1+0+3+4=8 (Integer solution x=2)
- If k=3,4k=12. f(3)=2+1+4+5=12 (Integer solution x=3)
- If k=4,4k=16. f(4)=3+2+5+6=16 (Integer solution x=4)
- If k=5,4k=20. f(5)=4+3+6+7=20 (Integer solution x=5)
- Matches: 2, 3, 4, 5
Part D: Differential Equation
- Equation: dxdy−y=1 (Linear D.E.)
- Integrating Factor (I.F.) =e∫−1dx=e−x
- Solution: ye−x=∫1⋅e−xdx=−e−x+C
- y=−1+Cex
Finding y(ln2)
- Apply y(0)=1: 1=−1+Ce0⟹C=2
- Particular solution: y=2ex−1
- Find y(ln2): y=2eln2−1=2(2)−1=3
- Matches: 3
Final Matching Summary
- Final Match:
- (A) → 1
- (B) → 2, 4
- (C) → 2, 3, 4, 5
- (D) → 3
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