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

Available for University of the Philippines Diliman via IEEE Xplore. Click here to access
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Schaum's outline of precalculus by Safier, Fred

Available for University of the Philippines Diliman via McGraw-Hill Access Engineering. Click here to access
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Algebra and trigonometry by Sullivan, Michael

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

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

Precalculus functions & graphs by Swokowski, Earl W.

Schaum's outline of trigonometry by 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 by Lorenzo, Carl F., Hartley, Tom T.

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

Plane & spherical trigonomerty a tetrahedron approach by Mamhot, Millard R., Endam, Joemar C., Mamhot, Alice A., Banagua, Emelyn C., Bantug, Jenette S.