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If the dependent variable y is changed t...

If the dependent variable y is changed to z by the substitution method y=tanz then the differential equation `d^2y/dx^2=1+2(1+y)/(1+y^2)(dy/dx)^2` is changed to `d^2z/dx^2=cos^2z+k(dz/dx)^2` then find the value off k

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The correct Answer is:
2
Given, `y=tanz`
`therefore (dy)/(dx)=sec^(2)z.(dz)/(dx)`………..(1)
Now, `(d^(2)y)/(dx^(2))=sec^(2)z.(d^(2)z)/(dx).d/(dx)(sec^(2)z)` [using product rule]
`=sec^(2)z. (d^(2)z)/(dx^(2))+(dz)/(dx).(d)/(dz)(sec^(2)z)(dz)/(dx)`
`(d^(2)y)/(dx^(2))=sec^(2)z.((dz)/(dx))^(2).2sec^(2)z.tanz`...........(2)
Now, `1+(2(1+y))/(1+y^(2))((dy)/(dx))^(2)`
=`1+(2(1+tanz))/(sec^(2)z).sec^(4)z.((dz)/(dx))^(2)`
`=1+2(1+tanz).sec^(2)z.((dz)/(dx))^(2)`
`=1+2sec^(2)z((dz)/(dx))^(2)+2tanz.sec^(2)z((dz)/(dx))^(2)`........(3)
From (2) and (3), we have RHS or (2) = RHS of (3)
or `sec^(2)z.(d^(2)z)/(dx^(2))=1+2sec^(2)z((dz)/(dx))^(2)`
or `(d^(2)z)/(dx^(2))=cos^(2)+2((dz)/(dx))^(2)`
or `k=2`
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