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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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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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. 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
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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

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

Worktext in college trigonometry por Domingo, Alleli C., Dimasuay, Lynie B., Lawas, Vernel M., Lubag, Sharon P., Almonte, Velimor D.

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