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Answers for “为什么视频说当根为实数时,解是 y = c_1 e^{r_1 t} + c_2 e^{r_2 t}?”

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Understand why

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When the characteristic roots r1r_1 and r2r_2 are real and distinct, each root generates a valid exponential solution er1te^{r_1 t} and er2te^{r_2 t}. Because the differential equation is linear and homogeneous, the superposition principle applies, meaning any linear combination of these solutions is also a solution.

Conditions: The differential equation is linear, homogeneous, and has constant coefficients.; The characteristic roots r1r_1 and r2r_2 are real and distinct.

Meet the concept

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The ±\pm symbol indicates that the quadratic formula produces two distinct roots for rr, denoted as r1r_1 and r2r_2. These two roots correspond to two independent exponential solutions, er1te^{r_1 t} and er2te^{r_2 t}, which form the basis of the general solution.

Conditions: The characteristic equation is quadratic.; The discriminant b2−4acb^2 - 4ac is positive (yielding two distinct real roots in the context of this specific clip's introduction).

Find a method

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The characteristic equation ar2+br+c=0ar^2 + br + c = 0 is solved by applying the quadratic formula. This yields the roots r=−b±b2−4ac2ar = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}, which provide the values for the parameter rr in the exponential trial solution.

Conditions: The characteristic equation is quadratic in rr.; a≠0a \neq 0.