ข้อสอบคณิต O-NET - 2552

$\mathrm{จากโจทย์}$

$$\begin{gathered} \sin A = \frac{\mathrm{ข้าม}}{\mathrm{ฉาก}} \\ \cos A = \frac{\mathrm{ชิด}}{\mathrm{ฉาก}} \\ \tan A = \frac{\mathrm{ข้าม}}{\mathrm{ชิด}} \end{gathered}$$

$$\begin{gathered} \mathrm{จะได้} \\ \sin 21° = \frac{BC}{AC} \\ \cos 21° = \frac{AB}{AC} \\ \tan 21° = \frac{BC}{AB} \\ \mathrm{จากโจทย์} \end{gathered}$$

$$\begin{gathered} \mathrm{จะได้} \\ \sin 69° = \frac{AB}{AC} \\ \cos 69° = \frac{BC}{AC} \\ \tan 69° = \frac{AB}{BC} \end{gathered}$$

$$\begin{gathered} \mathrm{ดังนั้น} \sin 21° = \cos 69° \\ \mathrm{ข้อ} 1 \mathrm{ถูก} \end{gathered}$$