f(x)=cosxf(x) = \cos xf(x)=cosx is concave up whereAsinx>0\sin x > 0sinx>0Bx>0x > 0x>0Csinx<0\sin x < 0sinx<0Dcosx<0\cos x < 0cosx<0check_circleExplanationf′′(x)=−cosx>0⇔cosx<0f''(x) = -\cos x > 0 \Leftrightarrow \cos x < 0f′′(x)=−cosx>0⇔cosx<0.