For θ+ϕ=90∘\theta + \phi = 90^\circθ+ϕ=90∘: sinθ=cosϕ\sin\theta = \cos\phisinθ=cosϕ, cosθ=sinϕ\cos\theta = \sin\phicosθ=sinϕ, tanθ=cotϕ\tan\theta = \cot\phitanθ=cotϕ, secθ=cscϕ\sec\theta = \csc\phisecθ=cscϕ. E.g. sin20∘=cos70∘\sin 20^\circ = \cos 70^\circsin20∘=cos70∘.