Kąt jest ostry oraz . Wtedy
cosα=15\displaystyle \cos\alpha= \frac{1}{5} cosα=51\displaystyle \cos\alpha= \frac{1}{5}
cosα=−15\displaystyle \cos\alpha= -\frac{1}{5} cosα=−51\displaystyle \cos\alpha= -\frac{1}{5}
cosα=−35\displaystyle \cos\alpha= -\frac{3}{5} cosα=−53\displaystyle \cos\alpha= -\frac{3}{5}
cosα=35\displaystyle \cos\alpha= \frac{3}{5} cosα=53\displaystyle \cos\alpha= \frac{3}{5}
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