diketahui ∆abc . panjang bc = 3 dan ac =5 cm jika sudut ABC =90° . maka nilai cosinus sudut BAC adalah..

→ Cosinus = Sisi samping sudut / sisi miring sudut
→ BC = 3 Cm
→ AC = 5 Cm
→ AB = ...Cm
→ ∠ABC = 90° (Siku - siku)
 
Cos ∠BAC = Cos ∠A
⇒ Cos ∠A = AB / AC
→ AB² = 5² - 3²
→ AB² = 16
→ AB = 4
⇒ Cos ∠A = 4/5 

Sin = depan / miring = y/r
Cos = samping / miring = x/r

Diketahui:
bc (a) = 3
ac (b) = 5
∠ABC (sinb) = 90° = 1

Ditanya:
Cos ∠ BAC = ??

Pembahasan:
a/sina = b/sinb
3/sina = 5/1
sina = (3 x 1)/5
sina = 3/5

x² = r² - y²
x² = 5² - 3²
x² = 25 - 9
x² = 16
x = √16
x = 4

Maka:
cosinus ∠ BAC:
= samping/miring
= x/r
= 4/5