1. NUMBERS
We already know that
𝒆𝒊𝜽 = 𝐜𝐨𝐬 𝜽 + 𝒊 𝒔𝒊𝒏 𝜽
𝒆−𝒊𝜽 = 𝒄𝒐𝒔 𝜽 − 𝒊 𝒔𝒊𝒏 𝜽
From the above two equations we have
𝐜𝐨𝐬 𝜽 =
𝒆𝒊𝜽 + 𝒆−𝒊𝜽
𝟐
& 𝐬𝐢𝐧 𝜽 =
𝒆𝒊𝜽 − 𝒆−𝒊𝜽
𝟐𝒊
These forms are known as Euler’s exponential forms of circular functions.
Further, if z is a complex number we define
𝒄𝒐𝒔 𝒛 =
𝒆𝒊𝒛
+ 𝒆−𝒊𝒛
𝟐
& 𝒔𝒊𝒏 𝒛 =
𝒆𝒊𝒛
− 𝒆−𝒊𝒛
𝟐𝒊
2. HYPERBOLIC FUNCTIONS
If x is real or complex
𝒆𝒙+𝒆−𝒙
𝟐
is called hyperbolic cosine of x and is denoted
by cosh x and
𝒆𝒙−𝒆−𝒙
𝟐
is called hyperbolic sine of x and is denoted by sinh x.
Thus, 𝒔𝒊𝒏𝒉 𝒙 =
𝒆𝒙−𝒆−𝒙
𝟐
, 𝒄𝒐𝒔𝒉 𝒙 =
𝒆𝒙+𝒆−𝒙
𝟐
Relationship between hyperbolic and circular functions:
(i) sin ix = i sinh x and sinh x = -i sin ix
(ii) cos ix = cosh x
(iii) tan ix = i tanh x and tanh x = -i tan ix
(iv) sinh ix = i sin x and sin x = -i sinh ix
(v) cosh ix = cos x
(vi) tanh ix = i tan x and tan x = -i tanh ix
8. INVERSE HYPERBOLIC FUNCTIONS
Definition: if sinh u = z then u is called inverse hyperbolic sine of z and is denoted
by 𝒖 = 𝒔𝒊𝒏𝒉−𝟏𝒛.
Similarly we can define inverse hyperbolic cosine and inverse hyperbolic tangent
and other functions which we denote as 𝒄𝒐𝒔𝒉−𝟏𝒛, 𝒕𝒂𝒏𝒉−𝟏𝒛, 𝒆𝒕𝒄.
The inverse hyperbolic functions are many valued but we will consider their
principal values only.
If Z is real we can show that
1. 𝒔𝒊𝒏𝒉−𝟏𝒛 = 𝒍𝒐𝒈 𝒛 + 𝒛𝟐 + 𝟏
2. 𝒄𝒐𝒔𝒉−𝟏
𝒛 = 𝒍𝒐𝒈 𝒛 + 𝒛𝟐 − 𝟏
3. 𝒕𝒂𝒏𝒉−𝟏
𝒛 =
𝟏
𝟐
𝒍𝒐𝒈
𝟏+𝒛
𝟏−𝒛
.
13. PROBLEMS BASED ON HYPERBOLIC FUNCTIONS
3. Solve the equation 𝒙𝟕
+ 𝒙𝟒
+ 𝒙𝟑
+ 𝟏 = 𝟎
Roots of a Complex Number: De Moivre’s Theorem can be used to find all n-
roots of a complex number.
Since, 𝒄𝒐𝒔 𝜽 = 𝒄𝒐𝒔 𝟐𝒌𝝅 + 𝜽 & 𝒔𝒊𝒏 𝜽 = 𝒔𝒊𝒏 (𝟐𝒌𝝅 + 𝜽) where k is an integer.
We have by DeMoivre’s Theorem
𝒄𝒐𝒔 𝜽 + 𝒊𝒔𝒊𝒏 𝜽
𝟏
𝒏 = 𝒄𝒐𝒔 𝟐𝒌𝝅 + 𝜽 + 𝒔𝒊𝒏 (𝟐𝒌𝝅 + 𝜽)
𝟏
𝒏
𝒄𝒐𝒔 𝜽 + 𝒊𝒔𝒊𝒏 𝜽
𝟏
𝒏 = 𝒄𝒐𝒔
𝟐𝝅𝒌 + 𝜽
𝒏
+ 𝒊𝒔𝒊𝒏
𝟐𝝅𝒌 + 𝜽
𝒏
By putting 𝒌 = 𝟎, 𝟏, 𝟐, … … . . 𝒏 − 𝟏 we get n roots of the complex number.