Correct Answer - Option 3 : (a
2 - 1)/(a
2 + 1)
Given:
Sec θ + Tan θ = a
Concept used:
Sec θ = 1/Cos θ
Cos θ = Base/Hypotenuse
Tan θ = Sin θ/Cos θ
Sin θ = Perpendicular/Hypotenuse
Tan2 θ + 1 = sec2 θ
Sec2 θ – Tan2 θ = 1
(Sec θ + Tan θ)(Sec θ – Tan θ) = 1
Sec θ – Tan θ = 1/(Sec θ + Tan θ))
Calculation:
Sec θ + Tan θ = a ---(I)
⇒ Sec θ - Tan θ = 1/a ---(II)
Adding (I) and (II);
2 Sec θ = a + (1/a)
⇒ 2/Cos θ = (a2 + 1)/a
⇒ 1/Cos θ = (a2 + 1)/2a
⇒ Cos θ = 2a/(a2 + 1)
Putting this value in a right angled triangle;
Cos θ = Base/Hypotenuse
Using Pythagoras theorem;
(H)2 = (B)2 + (P)2 ,
⇒ (a2 + 1)2 = (2a)2 + (P)2 ,
⇒ (P)2 = (a2 + 1)2 - (2a)2
⇒ (P)2 = a4 + 1 + 2a2 - 4a2
⇒ (P)2 = a4 + 1 - 2a2
⇒ (P)2 = (a2 - 1)2
⇒ P = (a2 - 1)
Sin θ = Perpendicular/Hypotenuse
⇒ (a2 - 1)/(a2 + 1)
∴ The value of Sin θ is (a2 - 1)/(a2 + 1)