42. Find the derivative of the following functions:

(i)sin x cos x  (ii)secx (iii)5 sec x + 4 cosx

(iv) cosecx (v)3cot x + 5 cosec x

(vi) 5sinx6cosx+7 (vii) 2tanx7secx

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  • A

    Answered by

    alok kumar singh | Contributor-Level 10

    3 months ago

    (i) f(x)=sin x cos x

    So, f?(x)=limh?0f(x+h)?f(x)h

    =limh?0sin(x+h)cos(x+h)?sinxcosxh

    =limh?012h*[2sin(x+h)cos(x+h)?2sinxcosx]

    =limh?012h[sin2(x+h)?sin2x]

    =limh?012h[2cos2(x+h)+2x2sin2(x+h)?2x2]

    =limh?01h[cos(2x+h)sinh]

    =limh?0cos(2x+h)*limh?0sinhh

    =cos(2x+0)

    =cos2x

    (ii) f(x)=secx

    So, f?(x)=limh?0f(x+h)?f(x)h

    =limh?01h[sec(x+h)?secx]

    =limh?01h[1cos(x+h)?1cosx]

    =limh?01h[cosx?cos(x+h)cos(x+h)cosx]

    =limh?01h[?2sin(x+x+h2)sin(x?(x+h)2)cos(x+h)cosx]

    =limh?01h[?2sin(2x+h2)sin(?h/2)cos(x+h)cosx]

    =limh?0(?1sin(2x+h2)cos(x+h)cosx*limh?0(?1)sinh/2h/2

    =sinxcosx?cosx*1

    =tanx?secx.

    (iii) Given f(x)=5 sec x+4 cosx.

    So, f?(x)=limh?0f(x+h)?f(x)h

    =limh?05sec(x+h)+4cos(x+h)?[5secx+4cosx]h.

    =limh?05h[sec(x+h)?secx]+limh?04h[cos(x+h)?cosx]

    =limh?05h[1cos(x+h)?1cosx]+limh?04h[?2sin(x+h+x2)sin(x+h?x2)]

    =limh?05h[cosx?cos(x+h)cos(x+h)(cosx)]+limh?04h[?2sin(2x+h2)sinh2]

    =limh?05h[?2sin(2x+h2)sin(?h/2)cos(x+h)cosx]?4limh?0sin(2x+h2)limh?0sinh/2h/2

    =sin(2x+02)cos(x+0)cosx*1?4sin(2x2)

    =5sinxcosx?1cosx?4sinx

    =5tanx?secx?4sinx

    (iv) Given f(x)=cosecx

    f?(x)=limh?0f(x+h)?f(x)h

    =limh?01h[cosec(x+h)?cosecx]

    =limh?01h[1sin(x+h)?1sinx]

    =limh?01h[sinx?sin(x+h)sin(x+h)sinx]

    =limh?01h[2cos(x+x+h2)sin(x?(x+h)2)]sin(x+h)sinx]

    =limh?01h[2cos(x+x+h2)sin(x?(x+h)2)sin(x+h)sinx]

    =limh?01h[2cos(2x+h2)sin(?h/2)]sin(x+h)sinx]

    =limh?0cos(2x+h2)sin(x+h)sinx*(?1)sin(2)h/2)

    =cos(2x+02)sin(x+0)sinx*(?1)

    =?cosxsinx*1sinx

    =?cotx?cosecx

    (v) Given,f(x)=3 cot x+5cosecx.

    So, f?(x)=limh?0f(x+h)?f(x)h =2cos(x+0)cosx*1+7sin(2x+02)*(?1)

    =limh?03cot(x+h)+5cosec(x+h)?[3cotx+5cosx)

    h?03h[cot(x+h)?cotx]+limh?0

    =?5h[cosec(x+h)?cosecx]

    =limh?03h[cos(x+h)sin(x+h)?cosxsinx]+limh?05h[1sin(x+h)?1sinx]

    =limh?03h[cos(x+h)sinx?cosxsin(x+h)sin(x+h)sinx]+limh?05h[sinx?sin(x+h)sin(x+h)sinx]

    =limh?03h[sin(x?(x+h))sin(x+h)sinx+limh?05h[2cos(x+x+h2)sin(x?(x+h)2)sin(x+h)sinx

    =limh?03sin(x+h)sinx*limh?0(?1)sinhh+limh?05h[2cos(2x+h2)sin(?h/2)sin(x+h)sinx

    =3sinx?sinx*(?1)+5limh

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