+118687 150 degrees is 2nd quad which means the cosine of it is negative.
Now just look at the acute angel equivalent 180-150=30degrees.
I do not memorise these ratios. I draw an equilateral triangle of side legth 2 and then bisect it like in the diagram.
I then worry only about one half of it. The bottom is 1 unit. The hypotenuse is 2 units. Using pythagoras' Theorum you get the height as sqrt(3)
$$Cos 150^0=\:-cos30^0=\frac{-\sqrt3}{2}$$
+129899 Well, you have to remember that this is just a 30° angle in the second quadrant. And the cos(30°) = √3/2.
But, since the cosine is negative in this quadrant, we have to take the negative value of this = -√3/2.
And that's it.....
+118687 150 degrees is 2nd quad which means the cosine of it is negative.
Now just look at the acute angel equivalent 180-150=30degrees.
I do not memorise these ratios. I draw an equilateral triangle of side legth 2 and then bisect it like in the diagram.
I then worry only about one half of it. The bottom is 1 unit. The hypotenuse is 2 units. Using pythagoras' Theorum you get the height as sqrt(3)
$$Cos 150^0=\:-cos30^0=\frac{-\sqrt3}{2}$$
