JEE MAIN - Mathematics (2020 - 7th January Morning Slot - No. 13)
If y = y(x) is the solution of the differential equation, $${e^y}\left( {{{dy} \over {dx}} - 1} \right) = {e^x}$$ such that y(0) = 0, then
y(1) is equal to:
2 + loge2
loge2
1 + loge2
2e
Explanation
Given,
$${e^y}\left( {{{dy} \over {dx}} - 1} \right) = {e^x}$$
$$ \Rightarrow $$ $${e^y}{{dy} \over {dx}} - {e^y} = {e^x}$$ ....(1)
Let $${e^y} = t$$ $$ \Rightarrow $$ $${e^y}{{dy} \over {dx}} = {{dt} \over {dx}}$$
$$ \therefore $$ $${{dt} \over {dx}} - t = {e^x}$$
So here p = -1 and q = ex
We know, IF = $${e^{\int {pdx} }}$$
= $${e^{\int { - 1dx} }}$$ = $${e^{ - x}}$$
$$ \therefore $$ t.$${e^{ - x}}$$ = $$\int {{e^x}.{e^{ - x}}dx} $$
$$ \Rightarrow $$ t.$${e^{ - x}}$$ = x + c
Putting value of t, we get
$${e^y}{e^{ - x}}$$ = x + c
$$ \Rightarrow $$ $${e^{y - x}} = x + c$$ .....(2)
Given y(0) = 0 means y = 0 when x = 0.
Putting in equation (2), we get
e0 = 0 + c
$$ \Rightarrow $$ c = 1
$$ \therefore $$ $${e^{y - x}} = x + 1$$ ....(3)
Now we have to find y(1), which means when x = 1 find the value of y in the above equation.
Putting x = 1 in equation (3)
$${e^{y - 1}} = 1 + 1$$
$$ \Rightarrow $$ y - 1 = loge2
$$ \Rightarrow $$ y = 1 + loge2
$$ \Rightarrow $$ $${e^y}{{dy} \over {dx}} - {e^y} = {e^x}$$ ....(1)
Let $${e^y} = t$$ $$ \Rightarrow $$ $${e^y}{{dy} \over {dx}} = {{dt} \over {dx}}$$
$$ \therefore $$ $${{dt} \over {dx}} - t = {e^x}$$
So here p = -1 and q = ex
We know, IF = $${e^{\int {pdx} }}$$
= $${e^{\int { - 1dx} }}$$ = $${e^{ - x}}$$
$$ \therefore $$ t.$${e^{ - x}}$$ = $$\int {{e^x}.{e^{ - x}}dx} $$
$$ \Rightarrow $$ t.$${e^{ - x}}$$ = x + c
Putting value of t, we get
$${e^y}{e^{ - x}}$$ = x + c
$$ \Rightarrow $$ $${e^{y - x}} = x + c$$ .....(2)
Given y(0) = 0 means y = 0 when x = 0.
Putting in equation (2), we get
e0 = 0 + c
$$ \Rightarrow $$ c = 1
$$ \therefore $$ $${e^{y - x}} = x + 1$$ ....(3)
Now we have to find y(1), which means when x = 1 find the value of y in the above equation.
Putting x = 1 in equation (3)
$${e^{y - 1}} = 1 + 1$$
$$ \Rightarrow $$ y - 1 = loge2
$$ \Rightarrow $$ y = 1 + loge2
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