Sine, cosine and tangent are just three different ratios of the same right triangle’s sides — knowing any one angle and any one side determines every other side and angle.
The mnemonic exists because all of trigonometry’s practical use — heights, distances, angles you can’t directly measure — reduces to picking the right one of these three ratios for what you already know.
Estimate with the rule, then check it against the calculator that models it properly.
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