"MATEMĀTIKA 10. KLASEI"
Formulas, kas divu leņķu summas vai starpības funkcijas izsaka ar šo leņķu funkcijām, sauc par argumentu saskaitīšanas formulām.

$\begin{array}{l}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\left(\mathrm{\alpha }+\mathrm{\beta }\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\phantom{\rule{0.147em}{0ex}}+\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\\ \mathrm{sin}\phantom{\rule{0.147em}{0ex}}\left(\mathrm{\alpha }-\mathrm{\beta }\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\phantom{\rule{0.147em}{0ex}}-\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\end{array}$
Ievēro, ka starp funkciju reizinājumiem ir tā pati zīme, kas starp argumentiem.

$\begin{array}{l}\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\left(\mathrm{\alpha }+\mathrm{\beta }\right)\phantom{\rule{0.147em}{0ex}}=\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\phantom{\rule{0.147em}{0ex}}-\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\\ \mathrm{cos}\phantom{\rule{0.147em}{0ex}}\left(\mathrm{\alpha }-\mathrm{\beta }\right)\phantom{\rule{0.147em}{0ex}}=\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\alpha }\phantom{\rule{0.147em}{0ex}}\mathrm{sin}\phantom{\rule{0.147em}{0ex}}\mathrm{\beta }\end{array}$
Ievēro, ka starp funkciju reizinājumiem ir pretējā zīme tai, kas ir starp argumentiem (pluss mainās uz mīnusu, bet mīnuss mainās uz plusu).

Piemērs:
Aprēķini $\mathrm{cos}15°$, nelietojot kalkulatoru un tabulas!
Vispirms ievērojam, ka $15°$ leņķi var uzrakstīt kā starpību no diviem leņķiem, kuru trigonometriskās funkcijas ir zināmas.
$\mathrm{cos}15°=\mathrm{cos}\phantom{\rule{0.147em}{0ex}}\left(45°-30°\right)$

Pielieto augstāk doto formulu:
$\begin{array}{l}\mathrm{cos}\left(45°-30°\right)=\\ =\mathrm{cos}45°\phantom{\rule{0.147em}{0ex}}\cdot \phantom{\rule{0.147em}{0ex}}\mathrm{cos}30°\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}\mathrm{sin}45°\phantom{\rule{0.147em}{0ex}}\cdot \phantom{\rule{0.147em}{0ex}}\mathrm{sin}30°=\\ =\frac{\sqrt{2}}{2}\cdot \frac{\sqrt{3}}{2}+\frac{\sqrt{2}}{2}\cdot \frac{1}{2}=\\ =\frac{\sqrt{6}+\sqrt{2}}{4}\end{array}$