John Napier

“ MUltiply the secant of the complement of the sum of the angles at the base by the tangent of half the base.
⁠Multiply the product by the sine of the greater angle at the base, and you will have the first found.
⁠Multiply the same product by the sine of the less angle, and you will have the second found. [d]
⁠Then divide the sum of the first and second found by the square of radius, and you will have the tangent of half the sum of the sides. ⁠Also subtract the less from the greater and you will have the tangent of half the difference of the sides. ”

“ From double this, namely 4357140, subtract 693147, and there will remain 3663993. The arc corresponding to this is 1° 28’, and the number put for the sine is 25595; but this is also the versed sine of 13°.⁠⁂
5.⁠Given two arcs, to find a third whose sine shall be equal to the sum of the sines of the given arcs. ”