1) Fundamental Identities:
a) sin2 ฮ + cos2 ฮ = 1
b) sec2 ฮ - tan2 ฮ = 1
c) csc2 ฮ - cot2 ฮ = 1
Proof:
a) sin2 ฮ + cos2 ฮ = 1
Here,
1) x2 + y2 = r2 -----------------(1)
Dividing equation 1 by r2 , we get,
2) (x2 / r2 + y2 / r2 = r2 / r2
3) (x / r) 2 + (y / r) 2 = 1 -------(2)
4) We know that,
y/r = sin ฮ
x/r = cos ฮ, ----- from equation 2, we have,
5) (cos ฮ) 2 + (sin ฮ) 2 = 1 ---(3)
6) (sin ฮ) 2 + (cos ฮ) 2 = 1
b) sec2 ฮ - tan2 ฮ = 1
Here,
1) x2 + y2 = r2 -------------------------(1)
Dividing equation 1 by x2,we get,
2) (x2 / x2 + y2 / x2 = r2 / x2
3) (x / x) 2 + (y / x) 2 = (r / x) 2 -------(2)
4) We know that,
y/x = tan ฮ
r/x = sec ฮ, ----- from equation 2, we have,
5) (1) 2 + (tan ฮ) 2 = (sec ฮ) 2 ---(3)
6) (sec ฮ) 2 - (tan ฮ) 2 = 1
c) csc2 ฮ - cot2 ฮ = 1
Here,
1) x2 + y2 = r2 -------------------------(1)
Dividing equation 1 by y2,we get,
2) (x2 / y2 + y2 / y2 = r2 / y2
3) (x / y) 2 + (y / y) 2 = (r / y) 2 -------(2)
4) We know that,
x/y = cot ฮ
r/y = csc ฮ, ----- from equation 2, we have,
5) (cot ฮ) 2 + (1) 2 = (csc ฮ) 2 ---(3)
6) (csc ฮ) 2 - (cot ฮ) 2 = 1