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**ally79** Sketch the graph of y=cos x in the range -pi < or equal to x < or equal to pi

Hence sketch the graph of y=|cos X| in the same range.

From my knowledge of sine and cosine waves i know what the graph will look like, and what the modulus graph will look like, what i dont know is how to prove it. i substituted pi and -pi into cosx and got -1 for both points on the graph, nbut i cant remember how t prove where the turning point is, i know its at (0,1) but i cant prove that without using prior knowledge,

Please help!