Let's find the value of sin 18°∙sin 54° without the calculator. Before we start... The double angle formula for sine function is: sin 2α = 2∙sin α∙cos α The co-function (0° ≤ α ≤ 90°) is: sin α = cos (90° - α) Or: cos α = sin (90° - α) This is a brain teaser for sum of angles in the trigonometric function , especially for sine (double-angle) — senior high school math. To solve sin 18°∙sin 54° , we can magically start from sin 72° Why 72°? 🤔 Because... 18° + 54° = 72° And... 72 ➗ 2 = 36 72 is 36 being doubled . It fits our theme, double-angle 💡 The magic is revealed 🪄 🧙♂️ We will see how the addition above will solve the problem. This is the image of the steps: The answer is ¼ or 0.25 . This method only works for 18° and 54°. Both sine functions. sin18°∙sin54° using co-function...