suppose u is an angle in the second quadrant and sin u=2/3.

i need to find cos u and tan u. and then what u is.

cos u= -sqrt (5/9)

tan u= (2/3)/ (-sqrt (5/9))

but i cant figure out what u is. i thought it would be just arc sin of 2/3 but i keep getting the wrong answer. oh and it wants the answer in radians.

my second question is cos (3x)=1/2. i need to find the two solutions between 0 and 120. i found that arc cos 1/2=3x so that x=20 and then i just guessed and found that the second answer is 100 degrees. but i dont know why....

i hope someone can help !! thanks a bunch!!