True or False? Explain your answer. If a triangle contains an obtuse angle, then it must be oblique. (2) True or False? Explain your answer. Because the sine function is an odd function, for a negative number u,sin2u=−2sinucosu (3) True or False? Explain your answer. If 2
π
​
<θ<π, then cos 2
θ
​
<0.