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Vector Algebra

Question
CBSEENMA12032956

Form the differential equation of the family of curves by eliminating arbitrary constants a and b.
y = ex (a cosx + b sinx)

Solution
The given equation is
       y = ex (a cosx + b sinx)                             ...(1)
Differentiating both sides w.r.t x, we get
                    dy over dx equals straight e to the power of straight x left parenthesis straight a space cosx plus space straight b space sinx right parenthesis space plus straight e to the power of straight x left parenthesis negative straight a space sinx space plus space straight b space cosx right parenthesis
or            dy over dx equals straight y plus straight e to the power of straight x left parenthesis negative straight a space sin space straight x space plus space straight b space cosx right parenthesis     ...(2)
                                                                               open square brackets because space of space left parenthesis 1 right parenthesis close square brackets
Again differentiating w.r.t x,
              fraction numerator straight d squared straight y over denominator dx squared end fraction equals space dy over dx plus straight e to the power of straight x left parenthesis negative straight a space sinx space plus space straight b space cosx right parenthesis space plus space straight e to the power of straight x left parenthesis negative straight a space cosx space minus space straight b space sinx right parenthesis
or        fraction numerator straight d squared straight y over denominator dx squared end fraction space equals space dy over dx plus open parentheses dy over dx minus straight y close parentheses space minus space straight e to the power of straight x left parenthesis straight a space cosx space plus space straight b space sinx right parenthesis space space space space space space space space space space space space space space space space open square brackets because space of space left parenthesis 2 right parenthesis close square brackets
or        fraction numerator straight d squared straight y over denominator dx squared end fraction space equals space dy over dx plus dy over dx minus straight y minus straight y                                                 open square brackets because space space of space space left parenthesis 1 right parenthesis close square brackets
or         fraction numerator straight d squared straight y over denominator dx squared end fraction minus 2 dy over dx plus 2 straight y equals 0, which is required differential equation.