A somewhat-funky Algebraic trick, which is interesting to me. Let's find the value of sin 18°∙sin 54° without the calculator. The things to remember are : The double angle formula for sine function, that is : sin 2α = 2∙sin α∙cos α The co-function (0° ≤ α ≤ 90°), that is : sin α = cos (90° - α) or cos α = sin (90° - α) This brain teaser is actually an exercise in the section of sum of angles within the trigonometric function, especially for sine (double-angle) - senior high Math. To "solve" sin 18°∙sin 54° , let's start from sin 72° . Why 72°? It's from 18° + 54° = 72° . You'll see why I just added those two angles as the first step. Look at this picture I made to watch where it will go : Pretty much fun . The answer is 1/4 or 0.25 . This method only works for 18° and 54°. Both sine functions. sin18°∙sin54° using co-function is equivalent with sin18°∙cos36° or cos72°∙sin54° or cos72°∙cos36° . Anyway, I also made a J...