If you try to graph cos(-π/4) you'll see that this is one side of a right isosceles triangle. If you draw a circle with radius 1 in the origin to get cosine and sine as x and y coordinates you can use a²+b²=c², and since c²=1=c in this case, and since a=b you get your answer.
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If you try to graph cos(-π/4) you'll see that this is one side of a right isosceles triangle. If you draw a circle with radius 1 in the origin to get cosine and sine as x and y coordinates you can use a²+b²=c², and since c²=1=c in this case, and since a=b you get your answer.