Question

Please help!! Why is sin(20) = sin(160) = -sin(200) = -sin(340)?

Answer 1

Explanation:

sin 20 is the same as sin 60 because 180-60=20. sin 20 is the same as -sin200 because 200-180=20

sin 340 is the same as sin 20 because 360-340=20. So all four of these connect and they all equal 20. Also, they all equal .3420 (rounded)

Answer 2

These relations all follow from the identity,

sin(x - y) = sin(x) cos(y) - cos(x) sin(y)

Notice that 160° = 180° - 20°, so if x = 180° and y = 20° above, we get

sin(160°) = sin(180°) cos(20°) - cos(180°) sin(20°)

We know sin(180°) = 0 and cos(180°) = -1, so we end up with

sin(160°) = sin(20°)

For the other, notice that 200° = 360° - 160°, and 340° = 360° - 20°.