The doctrine of triangles a history of modern trigonometry por Van Brummelen, Glen

Available for University of the Philippines Diliman via IEEE Xplore. Click here to access
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Algebra and trigonometry por Sullivan, Michael

Schaum's outline of precalculus por Safier, Fred

Available for University of the Philippines Diliman via McGraw-Hill Access Engineering. Click here to access
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Precalculus functions & graphs por Swokowski, Earl W.

Three-way journal writing effects on students' problem-solving and self-regulation skills in trigonometry por Lugtu, Ivy C.

Precalculus functions and graphs por Swokowski, Earl William 1926-, Cole, Jeffery A. (Jeffery Alan) 1955-

Schaum's outline of trigonometry por Moyer, Robert, Ayres, Frank 1901-1994

Available for University of the Philippines Diliman via McGraw-Hill Access Engineering. Click here to access
Also available remotely for University of the Philippines Diliman via McGraw-Hill Access Engineering. Click here to access thru EZproxy

The fractional trigonometry with applications to fractional differential equations and science por Lorenzo, Carl F., Hartley, Tom T.

A basic approach to precalculus trigonometry preparing to succeed in calculus por Kim, Young H., Kim, Youngsoo, Tameru, Ana, Yan, Wen

Trigonometry por Larson, Ron