- We need to evaluate
x→0limxsin2xαxex−βln(1+x)+γx2e−x=10.
Since the denominator tends to 0, for the limit to be finite, the numerator must vanish up to sufficiently high order.
- Expand each term near x=0.
We use:
ex=1+x+2x2+6x3+⋯,
e−x=1−x+2x2−6x3+⋯,
ln(1+x)=x−2x2+3x3−4x4+⋯,
sinx=x−6x3+⋯⟹sin2x=x2−3x4+⋯
Hence,
xsin2x=x3+O(x5).
Now expand the numerator:
αxex=αx(1+x+2x2+6x3+⋯)=αx+αx2+2αx3+6αx4+⋯
−βln(1+x)=−β(x−2x2+3x3−4x4+⋯)=−βx+2βx2−3βx3+4βx4+⋯
γx2e−x=γx2(1−x+2x2−6x3+⋯)=γx2−γx3+2γx4+⋯
Adding,
Numerator=(α−β)x+(α+2β+γ)x2+(2α−3β−γ)x3+⋯
- Since denominator is of order x3, for the limit to be finite we need coefficients of x and x2 in the numerator to be zero.
So,
α−β=0⇒α=β
and
α+2β+γ=0.
Using α=β,
α+2α+γ=0⇒γ=−23α.
- Now use the value of the limit.
Because
xsin2x∼x3,
the limit equals the coefficient of x3 in the numerator:
2α−3β−γ=10.
Since β=α and γ=−23α,
2α−3α+23α=10.
Combine terms:
2α+23α=2α,
so
2α−3α=10
36α−α=10
35α=10⇒α=6.
Hence,
β=6,
γ=−23⋅6=−9.
- Therefore,
α+β+γ=6+6−9=3.
So the required integer is
3.
- Comparison with stored answer:
Stored correct answer = 3.
Our derived answer also = 3.
Hence, they agree.