Prove that:
(i) $\frac{\sin (A+B)+\sin (A-B)}{\cos (A+B)+\cos (A-B)}=\tan A$
(ii) $\frac{\sin (A-B)}{\cos A \cos B}+\frac{\sin (B-C)}{\cos B \cos C}+\frac{\sin (C-A)}{\cos C \cos A}=0$
(iii) $\frac{\sin (A-B)}{\sin A \sin B}+\frac{\sin (B-C)}{\sin B \sin C}+\frac{\sin (C-A)}{\sin C \sin A}=0$
(iv) $\sin ^{2} B=\sin ^{2} A+\sin ^{2}(A-B)-2 \sin A \cos B \sin (A-B)$
(v) $\cos ^{2} A+\cos ^{2} B-2 \cos A \cos B \cos (A+B)=\sin ^{2}(A+B)$
(vi) $\frac{\tan (A+B)}{\cot (A-B)}=\frac{\tan ^{2} A-\tan ^{2} B}{1-\tan ^{2} A \tan ^{2} B}$
(i) LHS $=\frac{\sin (A+B)+\sin (A-B)}{\cos (A+B)+\cos (A-B)}$
$=\frac{\sin A \cos B+\cos A \sin B+\sin A \cos B-\cos A \sin B}{\cos A \cos B-\sin A \sin B+\cos A \cos B+\sin A \sin B}$
$=\frac{2 \sin A \cos B}{2 \cos A \cos B}$
$=\frac{\sin A}{\cos A}$
$=\tan A$
= RHS
Hence proved.
(ii) $\mathrm{LHS}=\frac{\sin (A-B)}{\cos A \cos B}+\frac{\sin (B-C)}{\cos B \cos C}+\frac{\sin (C-A)}{\cos C \cos A}$
$=\frac{\sin A \cos B-\cos A \sin B}{\cos A \cos B}+\frac{\sin B \cos C-\cos B \sin C}{\cos B \cos C}+\frac{\sin C \cos A-\cos C \sin A}{\cos C \cos A}$
$=\frac{\sin A \cos B}{\cos A \cos B}-\frac{\cos A \sin B}{\cos A \cos B}+\frac{\sin B \cos C}{\cos B \cos C}-\frac{\cos B \sin C}{\cos B \cos C}+\frac{\sin C \cos A}{\cos C \cos A}-\frac{\cos C \sin A}{\cos C \cos A}$
$=\frac{\sin A}{\cos A}-\frac{\sin B}{\cos B}+\frac{\sin B}{\cos B}-\frac{\sin C}{\cos C}+\frac{\sin C}{\cos C}-\frac{\sin A}{\cos A}$
$=\tan A-\tan B+\tan B-\tan C+\tan C-\tan A$
$=0$
= RHS
Hence proved.
(iii) $\mathrm{LHS}=\frac{\sin (A-B)}{\sin A \sin B}+\frac{\sin (B-C)}{\sin B \sin C}+\frac{\sin (C-A)}{\sin C \sin A}$
$=\frac{\sin A \cos B-\cos A \sin B}{\sin A \sin B}+\frac{\sin B \cos C-\cos B \sin C}{\sin B \sin C}+\frac{\sin C \cos A-\cos C \sin A}{\sin C \sin A}$
$=\frac{\sin A \cos B}{\sin A \sin B}-\frac{\cos A \sin B}{\sin A \sin B}+\frac{\sin B \cos C}{\sin B \sin C}-\frac{\cos B \sin C}{\sin B \sin C}+\frac{\sin C \cos A}{\sin C \sin A}-\frac{\cos C \sin A}{\sin C \sin A}$
$=\frac{\cos B}{\sin B}-\frac{\cos A}{\sin A}+\frac{\cos C}{\sin C}-\frac{\cos B}{\sin B}+\frac{\cos A}{\sin A}-\frac{\cos C}{\sin C}$
$=\cot B-\cot A+\cot C-\cot B+\cot A-\cot C$
$=0$
= RHS
Hence proved.
(iv) RHS $=\sin ^{2} A+\sin ^{2}(A-B)-2 \sin A \cos B \sin (A-B)$
$=\sin ^{2} A+\sin (A-B)\{\sin (A-B)-2 \sin A \cos B\}$
$=\sin ^{2} A+\sin (A-B)(\sin A \cos B-\cos A \sin B-2 \sin A \cos B)$
$=\sin ^{2} A+\sin (A-B)(-\sin A \cos B-\cos A \sin B)$
$=\sin ^{2} A-\sin (A-B)(\sin A \cos B+\cos A \sin B)$
$=\sin ^{2} A-\sin (A-B) \sin (A+B)$
$=\sin ^{2} A-\left(\sin ^{2} A-\sin ^{2} B\right)$
$=\sin ^{2} A-\sin ^{2} A+\sin ^{2} B$
$=\sin ^{2} B$
= RHS
Hence proved.
(v) LHS $=\cos ^{2} A+\cos ^{2} B-2 \cos A \cos B \cos (A+B)$
$=\cos ^{2} A+1-\sin ^{2} B-2 \cos A \cos B \cos (A+B)$
$=1+\cos ^{2} A-\sin ^{2} B-2 \cos A \cos B \cos (A+B)$
$=1+\cos ^{2} A-\sin ^{2} B-2 \cos A \cos B \cos (A+B)$
$=1+\cos (A+B) \cos (A-B)-2 \cos A \cos B \cos (A+B)$
$=1+\cos (A+B)\{\cos (A-B)-2 \cos A \cos B\}$
$=1+\cos (A+B)(\cos A \cos B+\sin A \sin B-2 \cos A \cos B)$
$=1+\cos (A+B)(-\cos A \cos B+\sin A \sin B)$
$=1-\cos (A+B)(\cos A \cos B-\sin A \sin B)$
$=1-\cos (A+B) \cos (A+B)$
$=1-\cos ^{2}(A+B)$
$=\sin ^{2}(A+B)$
= RHS
Hence proved.
(vi) $\mathrm{LHS}=\frac{\tan (A+B)}{\cot (A-B)}$
$=\frac{\tan (A+B)}{\frac{1}{\tan (A-B)}}$
$=\tan (A+B) \times \tan (A-B)$
$=\frac{\tan A+\tan B}{1-\tan A \tan B} \times \frac{\tan A-\tan B}{1+\tan A \tan B}$
$=\frac{(\tan A+\tan B)(\tan A-\tan B)}{(1-\tan A \tan B)(1+\tan A \tan B)}$
$=\frac{(\tan A)^{2}-(\tan B)^{2}}{(1)^{2}-(\tan A \tan B)^{2}}$
$=\frac{\tan ^{2} A-\tan ^{2} B}{1-\tan ^{2} A \tan ^{2} B}$
= RHS
Hence proved.
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All Study Material
- JEE Main
- Exam Pattern
- Previous Year Papers
- PYQ Chapterwise
- Physics
- Kinematics 1D
- Kinemetics 2D
- Friction
- Work, Power, Energy
- Centre of Mass and Collision
- Rotational Dynamics
- Gravitation
- Calorimetry
- Elasticity
- Thermal Expansion
- Heat Transfer
- Kinetic Theory of Gases
- Thermodynamics
- Simple Harmonic Motion
- Wave on String
- Sound waves
- Fluid Mechanics
- Electrostatics
- Current Electricity
- Capacitor
- Magnetism and Matter
- Electromagnetic Induction
- Atomic Structure
- Dual Nature of Matter
- Nuclear Physics
- Radioactivity
- Semiconductors
- Communication System
- Error in Measurement & instruments
- Alternating Current
- Electromagnetic Waves
- Wave Optics
- X-Rays
- All Subjects
- Physics
- Motion in a Plane
- Law of Motion
- Work, Energy and Power
- Systems of Particles and Rotational Motion
- Gravitation
- Mechanical Properties of Solids
- Mechanical Properties of Fluids
- Thermal Properties of matter
- Thermodynamics
- Kinetic Theory
- Oscillations
- Waves
- Electric Charge and Fields
- Electrostatic Potential and Capacitance
- Current Electricity
- Thermoelectric Effects of Electric Current
- Heating Effects of Electric Current
- Moving Charges and Magnetism
- Magnetism and Matter
- Electromagnetic Induction
- Alternating Current
- Electromagnetic Wave
- Ray Optics and Optical Instruments
- Wave Optics
- Dual Nature of Radiation and Matter
- Atoms
- Nuclei
- Semiconductor Electronics: Materials, Devices and Simple Circuits.
- Chemical Effects of Electric Current,