pi/3 is 60°, and the sine of that is √3 / 2. But yes, that's not something one calculates by head, that's memorization. And working out √3 / 2 into decimals a

pi/3 is 60°, and the sine of that is √3 / 2. But yes, that's not something one calculates by head, that's memorization. And working out √3 / 2 into decimals also takes a calculator, though I can approximate by knowing the first two significant digits would be 1.7 / 2 = 0.85. One can memorize trig tables like multiplication tables, and I do find that useful from time to time. I remember sin, cos, and tan for 0°, 30°, 45°, 60°, and 90°, and how to flip the signs to get all four quadrants from 0° to 360°.