Simple differential equation?

Calculus Level 5

If the solution of the equation

d y d x = y + 0 2 y d x \large{\dfrac{dy}{dx}=y+\displaystyle \int^{2}_{0} y \, dx}

is y ( x ) y(x) . And give that y ( 0 ) = 1 y(0) = 1 . Find the absolute value of y ( 2 ) \lceil y(2) \rceil .


The answer is 4.

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1 solution

Tanishq Varshney
Jun 25, 2015

let A = 0 2 y . d x A=\displaystyle \int^{2}_{0} y.dx

d y y + A = d x \large{\frac{dy}{y+A}=dx}

its solution is

l n ( y + A ) = x + c \large{ln(y+A)=x+c}

given y ( 0 ) = 1 y(0)=1 so c = l n ( 1 + A ) c=ln(1+A)

y = ( 1 + A ) e x A y=(1+A)e^{x}-A

A = 0 2 y . d x A=\displaystyle \int^{2}_{0} y.dx

A = 0 2 ( ( 1 + A ) e x A ) . d x \large {A=\displaystyle \int^{2}_{0} ((1+A)e^{x}-A).dx}

A = e 2 1 4 e 2 \large{A=\dfrac{e^{2}-1}{4-e^2}}

Hence solution is

y ( x ) = 3 e x e 2 + 1 4 e 2 \huge{y(x)=\dfrac{3e^x-e^2+1}{4-e^2}}

Excellent problem. Made silly mistakes each time and lost chances.

Vishwak Srinivasan - 5 years, 10 months ago

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Thanx ¨ \ddot \smile

Tanishq Varshney - 5 years, 10 months ago

Nice one! The first line is the most important part of the solution.

Vincent Miller Moral - 5 years, 10 months ago

nice solution ..

Pawan pal - 5 years, 1 month ago

How do we get the value of A?Can you explain a bit? @Tanishq Varshney

Anik Mandal - 4 years, 9 months ago

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By integrating and taking A as a constant.

Prithwish Mukherjee - 2 years ago

Did the same

Siddharth Yadav - 4 years, 4 months ago

why isnt integral 0 2 of ydx just 2y?

<> <> - 3 years, 10 months ago

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because y is a function of x. Think about it . Is integral of f(x) or g(x) from 0 to 2 is 2 ? . Then why should it be different for y.

Arghyadeep Chatterjee - 3 years, 2 months ago

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Yes , y is not a constant in that integration as y =f (x)

Prithwish Mukherjee - 2 years ago

@Tanishq Varshney I think the answer to question has an error Since you asked the absolute value of the integral part and not the integral part of the absolute value the answer should be 5 Not 4

Ahindra Kandarpa - 1 year, 10 months ago

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