Suppose $y=y(x)$ be the solution curve to the differential equation $\frac{d y}{d x}-y=2-e^{-x}$ such that $\lim\limits_{x \rightarrow \infty} y(x)$ is finite. If $a$ and $b$ are respectively the $x$ - and $y$-intercepts of the tangent to the curve at $x=0$, then the value of $a-4 b$ is equal to _____________.
Answer (integer)
3
Solution
<p>IF $= {e^{-x}}$</p>
<p>$y\,.\,{e^{-x}} = - 2{e^{ - x}} + {{{e^{ - 2x}}} \over 2} + C$</p>
<p>$\Rightarrow y = - 2 + {e^{ - x}} + C{e^x}$</p>
<p>$\mathop {\lim }\limits_{x \to \infty } \,y(x)$ is finite so $C = 0$</p>
<p>$y = - 2 + {e^{ - x}}$</p>
<p>$$ \Rightarrow {\left. {{{dy} \over {dx}} = - {e^{ - x}} \Rightarrow {{dy} \over {dx}}} \right|_{x = 0}} = - 1$$</p>
<p>Equation of tangent</p>
<p>$y + 1 = - 1(x - 0)$</p>
<p>or $y + x = - 1$</p>
<p>So $a = - 1,\,b = - 1$</p>
<p>$\Rightarrow a - 4b = 3$</p>
About this question
Subject: Mathematics · Chapter: Differential Equations · Topic: Order and Degree
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