Answer:
10 units
Step-by-step explanation:
Given that:
Cos a = 3 /5
From trigonometry ;
Cos θ = Adjacent / hypotenus
Side Adjacent angle a = 3 = FD
Hypotenus = 5 = DE
3 / 5 = FD / DE
FD = 6 units
3/5 = 6/DE
DE * 3/5 = 6
DE = 6 ÷ 3/5
DE = 6 * 5 /3
DE = 30 /3
DE = 10 units
