Question

If the graph of y=f(x) passes through the point (0,1), and dy/dx=xsin(x^2)/y, then f(x)= ?

Answer 1

The differential equation

dy/dx = x sin(x ²) / y

is separable as

y dy = x sin(x ²) dx

Integrate both sides:

∫ y dy = ∫ x sin(x ²) dx

∫ y dy = 1/2 ∫ 2x sin(x ²) dx

∫ y dy = 1/2 ∫ sin(x ²) d(x ²)

1/2 y ² = -1/2 cos(x ²) + C

Solve for y implicitly:

y ² = -cos(x ²) + C

Given that y = 1 when x = 0, we get

1² = -cos(0²) + C

1 = -cos(0) + C

1 = -1 + C

C = 2

Then the particular solution to the DE is

y ² = 2 - cos(x ²)

Solving explicitly for y would give two solutions,

y = ± √(2 - cos(x ²))

but only the one with the positive square root satisfies the initial condition:

√(2 - cos(0²)) = √1 = 1

-√(2 - cos(0²)) = -√1 = -1 ≠ 1

So the unique solution to this DE is

y = √(2 - cos(x ²))