The doctrine of triangles a history of modern trigonometry Bằng 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 Bằng 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 Bằng Lugtu, Ivy C.

Precalculus functions and graphs Bằng Swokowski, Earl William 1926-, Cole, Jeffery A. 1955-

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

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

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

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

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

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