Identitas trigonometri (sin delta - cos delta)^2= 1-2 sin delta cos delta

(sin ∆ - cos ∆)²
= sin² ∆ + cos² ∆ - 2 sin ∆ cos ∆
= 1 - 2 sin ∆ cos ∆
q.e.d

Trigonometry.

(a - b)² = a² - 2ab + b²

(sin Δ - cos Δ)
= sin² Δ - 2 sin Δ cos Δ + cos² Δ
= 1 - 2 sin Δ cos Δ
Proved.