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

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Algebra and trigonometry af Sullivan, Michael

Schaum's outline of precalculus af 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 af Swokowski, Earl W.

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

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

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

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

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

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