Question:

Prove \(\tan^{-1}\frac{2}{11}+\tan^{-1}\frac{7}{24}=\tan^{-1}\frac{1}{2}\)

Updated On: Aug 26, 2023
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Solution and Explanation

To prove: \(\tan^{-1}\frac{2}{11}+\tan^{-1}\frac{7}{24}=\tan^{-1}\frac{1}{2}\)

LHS= \(\tan^{-1}\frac{2}{11}+\tan^{-1}\frac{7}{24}\)

= \(\tan^{-1}\frac{\frac{2}{11}+\frac{7}{24}}{1-\frac{2}{11}+\frac{7}{24}}\)                                            \(\bigg[\tan^{-1}x+\tan^{-1}y=\tan^{-1}\frac{x+y}{1-xy}\bigg]\)

= \(\tan^{-1}\frac{48-77}{264-14}=\tan^{-1}\frac{125}{250}=\tan^{-1}\frac{1}{2}=R.H.S.\)

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Notes on Inverse Trigonometric Functions

Concepts Used:

Properties of Inverse Trigonometric Functions

The elementary properties of inverse trigonometric functions will help to solve problems. Here are a few important properties related to inverse trigonometric functions:

Property Set 1:

  • Sin−1(x) = cosec−1(1/x), x∈ [−1,1]−{0}
  • Cos−1(x) = sec−1(1/x), x ∈ [−1,1]−{0}
  • Tan−1(x) = cot−1(1/x), if x > 0  (or)  cot−1(1/x) −π, if x < 0
  • Cot−1(x) = tan−1(1/x), if x > 0 (or) tan−1(1/x) + Ï€, if x < 0

Property Set 2:

  • Sin−1(−x) = −Sin−1(x)
  • Tan−1(−x) = −Tan−1(x)
  • Cos−1(−x) = Ï€ − Cos−1(x)
  • Cosec−1(−x) = − Cosec−1(x)
  • Sec−1(−x) = Ï€ − Sec−1(x)
  • Cot−1(−x) = Ï€ − Cot−1(x)

Property Set 3:

  • Sin−1(1/x) = cosec−1x, x≥1 or x≤−1
  • Cos−1(1/x) = sec−1x, x≥1 or x≤−1
  • Tan−1(1/x) = −π + cot−1(x)

Property Set 4:

  • Sin−1(cos θ) = Ï€/2 − θ, if θ∈[0,Ï€]
  • Cos−1(sin θ) = Ï€/2 − θ, if θ∈[−π/2, Ï€/2]
  • Tan−1(cot θ) = Ï€/2 − θ, θ∈[0,Ï€]
  • Cot−1(tan θ) = Ï€/2 − θ, θ∈[−π/2, Ï€/2]
  • Sec−1(cosec θ) = Ï€/2 − θ, θ∈[−π/2, 0]∪[0, Ï€/2]
  • Cosec−1(sec θ) = Ï€/2 − θ, θ∈[0,Ï€]−{Ï€/2}
  • Sin−1(x) = cos−1[√(1−x2)], 0≤x≤1 = −cos−1[√(1−x2)], −1≤x<0

Property Set 5:

  • Sin−1x + Cos−1x = Ï€/2
  • Tan−1x + Cot−1(x) = Ï€/2
  • Sec−1x + Cosec−1x = Ï€/2

Property Set 6:

  • If x, y > 0

Tan−1x + Tan−1y = π + tan−1 (x+y/ 1-xy), if xy > 1

Tan−1x + Tan−1y = tan−1 (x+y/ 1-xy), if xy < 1

  • If x, y < 0

Tan−1x + Tan−1y = tan−1 (x+y/ 1-xy), if xy < 1

Tan−1x + Tan−1y = -π + tan−1 (x+y/ 1-xy), if xy > 1

Property Set 7:

  • sin−1(x) + sin−1(y) = sin−1[x√(1−y2)+ y√(1−x2)]
  • cos−1x + cos−1y = cos−1[xy−√(1−x2)√(1−y2)]

Property Set 8:

  • sin−1(sin x) = −π−π, if x∈[−3Ï€/2, −π/2]

= x, if x∈[−π/2, π/2]

= π−x, if x∈[π/2, 3π/2]

=−2π+x, if x∈[3π/2, 5π/2] And so on.

  • cos−1(cos x) = 2Ï€+x, if x∈[−2Ï€,−π]

= −x, ∈[−π,0]

= x, ∈[0,π]

= 2π−x, ∈[π,2π]

=−2π+x, ∈[2π,3π]

  • tan−1(tan x) = Ï€+x, x∈(−3Ï€/2, −π/2)

= x, (−π/2, π/2)

= x−π, (π/2, 3π/2)

= x−2π, (3π/2, 5π/2)

Property Set 9: