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Question

The maximum value of the solution y(t) of the differential equation y(t)+..y(t)=0 with the initial conditions .y(0)=1 and y(0)=1, for t0 is

A
1
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B
2
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C
π
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D
2
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Solution

The correct option is D 2
Given ..y(t)+y(t)=0
(D2+1)y=0 ... (1)
A.E is m2+1=0m=±i
The general solution of equation (1) is
y=C1cost+C2sint
Using y(0)=1 & .y(0)=1, we get C1=1 and C2=1
The solution of equation (1) is
y=cost+sint ...(2)
For maximum (or) minimum of equation (2), we have
dydt=0
sint+cost=0 or tant=1t=π4
Hence, the maximum value of equation (2) is
y(π4)=cos(π4)+sin(π4)=2

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