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Sagot :
- a) Finding the derivatives and replacing into the equation, we reach an identity, an thus, for any value of the constants, [tex]y = Ae^{-7x} + Be^{\frac{x}{2}}[/tex] is a solution.
- b) Solving a system of equations, according to the conditions, we find that: [tex]A = -\frac{7}{15}, B = \frac{52}{15}[/tex]
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Just a correction, the characteristic roots of the equation are [tex]y = 7[/tex] and [tex]y = -\frac{1}{2}[/tex], thus, we should test for:
[tex]y = Ae^{7x} + Be^{-\frac{x}{2}}[/tex]
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Question a:
- First, we find the derivatives, thus:
[tex]y = Ae^{7x} + Be^{-\frac{x}{2}}[/tex]
[tex]y^{\prime} = 7Ae^{-7x} - \frac{1}{2}Be^{-\frac{x}{2}}[/tex]
[tex]y^{\prime\prime} = 49Ae^{-7x} + \frac{1}{4}Be^{-\frac{x}{2}}[/tex]
- Now, we replace into the equation:
[tex]2y^{\prime\prime} - 13y^{\prime} - 7y = 0[/tex]
[tex]2(49Ae^{-7x} + \frac{1}{4}Be^{-\frac{x}{2}}) - 13(7Ae^{-7x} - \frac{1}{2}Be^{-\frac{x}{2}}) - 7(Ae^{7x} + Be^{-\frac{x}{2}}) = 0[/tex]
[tex]98Ae^{-7x} + \frac{1}{2}Be^{\frac{x}{2}} - 91Ae^{-7x} + \frac{13}{2}e^{-\frac{x}{2}} - 7Ae^{7x} - 7Be^{-\frac{x}{2}} = 0[/tex]
[tex]98Ae^{-7x} - 91Ae^{-7x} - 7Be^{-\frac{x}{2}} + \frac{1}{2}Be^{\frac{x}{2}} + \frac{13}{2}e^{-\frac{x}{2}} - 7Be^{-\frac{x}{2}} = 0[/tex]
[tex]0A + 0B = 0[/tex]
[tex]0 = 0[/tex], thus, we found the identity, and for each constant A and B, the following is a solution.
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Question b:
[tex]y = Ae^{7x} + Be^{-\frac{x}{2}}[/tex]
- Since [tex]y(0) = 3[/tex]
[tex]A + B = 3 \rightarrow B = 3 - A[/tex]
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[tex]y^{\prime} = 7Ae^{-7x} - \frac{1}{2}Be^{-\frac{x}{2}}[/tex]
- Since [tex]y^{\prime}(0) = -5[/tex]
[tex]7A - \frac{1}{2}B = -5[/tex]
Using [tex]B = 3 - A[/tex]
[tex]7A - \frac{3}{2} + \frac{A}{2} = -5[/tex]
[tex]\frac{14A}{2} + \frac{A}{2} = -\frac{10}{2} + \frac{3}{2}[/tex]
[tex]\frac{15A}{2} = -\frac{7}{2}[/tex]
[tex]15A = -7[/tex]
[tex]A = -\frac{7}{15}[/tex]
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Then, B is given by:
[tex]B = 3 - A = 3 - (-\frac{7}{15}) = \frac{45}{15} + \frac{7}{15} = \frac{52}{15}[/tex]
Thus, the values are: [tex]A = -\frac{7}{15}, B = \frac{52}{15}[/tex]
A similar problem is given at https://brainly.com/question/2456414
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