Segitiga DEF mempunyai panjang sisi DE = 3, EF = 7cm,dan DF = 5 cm.Nilai cos F =​

trigonometri

aturan cosinus

∆DEF

DE² = DF² + FE² – 2.DF.FE cos F

cos F

= (DF² + FE² – DE²) / (2.DF.FE)

= (5² + 7² – 3²) / (2.5.7)

= (25 + 49 – 9) / 2.5.7

= 65 / 5.14

= 13/14

Nilai cos F = 13/14