Trigonometric Ratios of Sums of Two Angles

Proof 1

\( \sin (\alpha + \beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta \)

\( \begin{align} \angle RPN &= 90^{\circ} – \angle PNR \\ &= \angle RNO \\ &= \angle RNO \\ &= \angle NOQ \\ &= \alpha \\ \sin (\alpha + \beta) &= \sin \angle AOC \\ &= \displaystyle \frac{MP}{OP} \\ &= \frac{MR + RP}{OP} \\ &= \frac{QN}{OP} + \frac{RP}{OP} \\ &= \frac{QN}{\color{red}{ON}} \times \frac{\color{red}{ON}}{OP} + \frac{RP}{\color{red}{NP}} \times \frac{\color{red}{NP}}{OP} \\ &= \sin \alpha \cos \beta + \cos \angle RPN \sin \beta \\ \therefore \sin (\alpha + \beta) &= \sin \alpha \cos \beta + \cos \alpha \sin \beta \end{align} \)

Proof 2

\( \cos (\alpha + \beta) = \cos \alpha \cos \beta – \sin \alpha \sin \beta \)

Trigonometric-Ratios-of-Sums-of-Two-Angles

\( \begin{align} \cos (\alpha + \beta) &= \cos \angle AOC \\ &= \displaystyle \frac{OM}{OP} \\ &= \frac{OQ-MQ}{OP} \\ &= \frac{OQ}{OP} – \frac{RN}{OP} \\ &= \frac{OQ}{\color{red}{ON}} \times \frac{\color{red}{ON}}{OP} – \frac{RN}{\color{red}{NP}} \times \frac{\color{red}{NP}}{OP} \\ &= \cos \alpha \cos \beta – \sin \angle RPN \sin \beta \\ \therefore \cos (\alpha + \beta) &= \cos \alpha \cos \beta – \sin \alpha \sin \beta \end{align} \)

Proof 3

\( \tan (\alpha + \beta) = \displaystyle \frac{\tan \alpha + \tan \beta}{1 – \tan \alpha \tan \beta} \)

\( \begin{align} \tan (\alpha + \beta) &= \displaystyle \frac{\sin (\alpha + \beta)}{\cos (\alpha + \beta)} \\ &= \frac{\sin \alpha \cos \beta + \cos \alpha \sin \beta}{\cos \alpha \cos \beta – \sin \alpha \sin \beta} \\ &= \frac{\displaystyle \frac{\sin \alpha \cos \beta}{\color{red}{\cos \alpha \cos \beta}} + \frac{\displaystyle \cos \alpha \sin \beta}{\color{red}{\cos \alpha \cos \beta}}}{\displaystyle \frac{\cos \alpha \cos \beta}{\color{red}{\cos \alpha \cos \beta}} – \frac{\displaystyle \sin \alpha \sin \beta}{\color{red}{\cos \alpha \cos \beta}}} \\ &= \frac{\displaystyle \frac{\sin \alpha}{\cos \alpha}+\frac{\sin \beta}{\cos \beta}}{1- \displaystyle \frac{\sin \alpha}{\cos \alpha} \times \frac{\sin \beta}{\cos \beta}} \\ &= \displaystyle \frac{\tan \alpha + \tan \beta}{1 – \tan \alpha \tan \beta} \\ \therefore \tan (\alpha + \beta) &= \displaystyle \frac{\tan \alpha + \tan \beta}{1 – \tan \alpha \tan \beta} \end{align} \)

Example 1

Find the exact value of \( \sin 75^{\circ} \).

\( \begin{align} \sin 75 ^{\circ} &= \sin (45^{\circ} + 30^{\circ}) \\ &= \sin 45^{\circ} \cos 30^{\circ} + \cos 45^{\circ} \sin 30^{\circ} \\ &= \displaystyle \frac{1}{\sqrt{2}} \times \frac{\sqrt{3}}{2} + \frac{1}{\sqrt{2}} \times \frac{1}{2} \\ &= \frac{\sqrt{3}+1}{2 \sqrt{2}} \end{align} \)

Example 2

Find the exact value of \( \cos 75^{\circ} \).

\( \begin{align} \cos 75 ^{\circ} &= \cos (45^{\circ} + 30^{\circ}) \\ &= \cos 45^{\circ} \cos 30^{\circ} – \sin 45^{\circ} \sin 30^{\circ} \\ &= \displaystyle \frac{1}{\sqrt{2}} \times \frac{\sqrt{3}}{2} – \frac{1}{\sqrt{2}} \times \frac{1}{2} \\ &= \frac{\sqrt{3}-1}{2 \sqrt{2}} \end{align} \)

Example 3

Find the exact value of \( \tan 75^{\circ} \).

\( \begin{align} \tan 75 ^{\circ} &= \tan (45^{\circ} + 30^{\circ}) \\ &= \displaystyle \frac{\tan 45^{\circ} + \tan 30^{\circ}}{1 – \tan 45^{\circ} \times \tan 30 ^{\circ}} \\ &= \frac{1 + \displaystyle \frac{1}{\sqrt{3}}}{1 – \displaystyle 1 \times \frac{1}{\sqrt{3}}} \\ &= \frac{\sqrt{3}+1}{\sqrt{3}-1} \\ &= \frac{(\sqrt{3}+1)^2}{3-1} \\ &= \frac{4+2\sqrt{3}}{2} \\ &= 2+\sqrt{3} \end{align} \)

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