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Derivatives of Algebraic Expressions


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Overview

In this page, Derivatives of the following are explained.

 →  standard functions in algebraic expressions

 →  trigonometric functions sin, cos, tan etc.

 →  inverse trigonometric functions such as arcsin, arccos, etc.

 →  exponents and logarithmic functions such as ex, ax, and lnx

constant

Finding the derivative of y=a in first principles:

dydx

=limδ0[f(x+δ)-f(x)]/δ

=limδ0[a-a]/δ

=limδ0[0]/δ

The above proves
ddx(a)=0
Note that δ is very small value and the numerator is 0. So the limit evaluates to 0.

exponent

Finding the derivative of y=xn in first principles:

dydx

=limδ0[f(x+δ)-f(x)]/δ

=limδ0[(x+δ)n-xn]/δ

using binomial expression for (x+δ)n =limδ0[(xn+nδxn-1+nC2δ2xn-2++δn)-xn]/δ

canceling xn and -xn
=limδ0[(nδxn-1+nC2δ2xn-2++δn)]/δ

dividing all terms by δ
=limδ0[(nxn-1+nC2δ1xn-2++δn-1)]

applying limit δ0
The δ is substituted as 0 and only one term is non zero. The above proves
ddx(xn)=nxn-1".

summary

Derivatives of Algebraic Expressions :
  Derivative of a constant
   ddxa=0
rate of change of constant is 0

  Derivative of power:
   ddxxn=nxn-1
small change results in binomial expansion and only one factor remains

example

Find the derivative of x3+4x2-3x-2
The answer is "3x2+8x-3"

sine

Finding the derivative of y=sinx in first principles:

dydx

=limδ0[f(x+δ)-f(x)]/δ

=limδ0[sin(x+δ)-sinx]/δ

=limδ0[sinxcosδ+cosxsinδ-sinx]/δ

=limδ0cosxsinδδ-limδ0sinx-sinxcosδδ

=cosxlimδ0sinδδ-sinxlimδ01-cosδδ

applying the standard limits
=cosx×1-sinx×0

The above proves
ddxsinx=cosx

cosine

Finding the derivative of y=cosx:

ddxcosx

=ddxsin(π2-x)

applying chain rule of differentiation with u=π2-x
=ddusin(u)ddx(π2-x)

=cos(u)×(-1)

=-cos(π2-x)

=-sin(x)

The above proves
ddxcosx=-sinx

tangent

Finding the derivative of y=tanx

(tanx)

=(sinxcosx)

applying quotient rule of differentiation
=cosx(sinx)-sinx(cosx)cos2x

=cosx(cosx)-sinx(-sinx)cos2x

=cos2x+sin2xcos2x

=1cos2x

=sec2(x)

The above proves
ddxtanx=sec2x

cotangent

Finding the derivative of y=cotx

(cotx)

=(cosxsinx)

applying quotient rule of differentiation
=sinx(cosx)-cosx(sinx)sin2x

=sinx(-sinx)-cosx(cosx)sin2x

=-sin2x-cos2xsin2x

=-1sin2x

=-csc2(x)

The above proves
ddxcotx=-csc2x

secant

Finding the derivative of y=secx

(secx)

=(1cosx)

applying quotient rule of differentiation
=cosx(1)-(cosx)cos2x

=cosx(0)-(-sinx)cos2x

=sinxcos2x

=secxtanx

The above proves
ddxsecx=secxtanx

cosecant

Finding the derivative of y=cscx

(cscx)

=(1sinx)

applying quotient rule of differentiation
=sinx(1)-(sinx)sin2x

=sinx(0)-(cosx)sin2x

=-cosxsin2x

=-cscxcotx

The above proves
ddxcscx=-cscxcotx

summary

Derivatives of Trigonometric Functions:
  ddxsinx=cosx

  ddxcosx=-sinx
cosx=sin(π2-x) and the negative sign of x is carried in the result

  ddxtanx=sec2x
tan=sincos and results in 1cos2

  ddxcotx=-csc2x
cot=tan(π2-x) and the negative sign of x is carried

  ddxsecx=secxtanx
sec=1cos and results in sincos2

  ddxcscx=-cscxcotx
csc=sec(π2-x) and the negative sign of x is carried

example

Find the derivative of tanx-x
The answer is "tan2x".

tanx-x
sec2x-1
tan2x

inverse sine

Finding the derivative of y=arcsinx:
y=arcsinx
siny=x
differentiate this
cosydydx=1

dydx=1cosy
=11-sin2y
=11-x2

The above proves
ddxarcsinx=11-x2

inverse cosine

Finding the derivative of y=arccosx:
y=arccosx
cosy=x
differentiate this
-sinydydx=1

dydx=1-siny
dydx=-11-cos2y
dydx=-11-x2

The above proves
ddxarccosx=-11-x2

inverse tangent

Finding the derivative of y=arctanx:
y=arctanx
tany=x
differentiate this
sec2ydydx=1

dydx=1sec2y
=11+tan2y
=11+x2

The above proves
ddxarctanx=11+x2

inverse secant

Finding the derivative of y=arcsecx:
y=arcsecx
y is in first or second quadrant.
secy=x
differentiate this
secytanydydx=1
dydx=1secytany
secytany is always positive for values in first and second quadrant.
dydx=1secysec2y-1
=1|x|x2-1

The above proves
ddxarcsecx=1|x|x2-1

inverse cosecant

Finding the derivative of y=arccscx:
y=arccscx
y is in range -π2yπ2.
cscy=x
differentiate this
-cscycotydydx=1

dydx=-1cscycoty
cscycoty is always positive for values in range -π2yπ2
=-1cscycsc2y-1
=-1|x|x2-1

The above proves
ddxarccscx=-1|x|x2-1

inverse cotangent

Finding the derivative of y=arccotx:
y=arccotx
coty=x
differentiate this
-csc2ydydx=1

dydx=-1csc2y
=-11+tan2y
=-11+x2

The above proves
ddxarccotx=-11+x2

summary

Derivatives of inverse trigonometric functions : trig fn(y)=x, so derivative in terms of x

ddxarcsinx=11-x2

ddxarccosx=-11-x2

ddxarctanx=11+x2

ddxarcsecx=1|x|x2-1

ddxarccscx=-1|x|x2-1

ddxarccotx=-11+x2

example

Find the derivative of (1+x2)arctanx
The answer is "1+2xarctanx"

[(1+x2)arctanx]
(1+x2)(arctanx)+(1+x2)arctanx
(1+x2)(11+x2)+2xarctanx
1+2xarctanx

exponent of e

Finding the derivative of y=ex in first principles:

ddxex

=limδ0[ex+δ-ex]/δ

=limδ0[ex×eδ-ex]/δ

=limδ0ex[eδ-1]/δ

applying the standard limit limp0(ep-1)/p=1

=ex

The above proves
ddxex=ex

logarithm base e

Finding the derivative of y=lnx :

y=lnx

ey=x

differentiating the equation
ddxey=1

applying chain rule ddyeydydx=1

eydydx=1

xdydx=1

dydx=1x

The above proves
ddxlnx=1x

exponent

Finding the derivative of y=ax :

substituting a=elna

y=exlna

differentiating the equation
dydx=ddxexlna

applying chain rule with u=xlna
dydx=ddueuddx(xlna)

dydx=eu×lna

dydx=exlna×lna

substituting elna=a

dydx=axlna

The above proves
ddxax=axlna

summary

  ddxex=ex
definition of e is rate of change is proportional to itself

  ddxax=axlna
a equals elna

  ddxlnx=1x
natural log is inverse of e power

example

What is the derivative of log10x?
Note: Use the identity log10x=logexloge10

The answer is "1xln10".
ddxlog10x
=ddxlogexloge10
=1loge10ddxlogex
=1loge10×1x
=1xloge10


What is the derivative of eax?

The answer is "aeax". Applying chain rule with u=ax,
ddueuddxax
eu×a
aeax

Outline

The outline of material to learn "Differential Calculus" is as follows.

•   Detailed outline of Differential Calculus

    →   Application Scenario

    →   Differentiation in First Principles

    →   Graphical Meaning of Differentiation

    →   Differntiability

    →   Algebra of Derivatives

    →   Standard Results