Why does cos(t)=x and sin(t)=y?

Why cos(-x)= cosx?

Cos of minus theta is equal to cos theta: proof | Cos(-θ) = Cos(θ) Proof

cos(x)=x

Derivative of Cos x from First Principles.

Proof that the Derivative of sin(x) is cos(x) using the Limit Definition of the Derivative

cos(-theta)=cos theta proof | cos minus theta is equal to cos theta

Derivative of sin(x) and cos(x), PROOF

Derivative of sin x and cos x

Proof that the Derivative of cos(x) is -sin(x) using the Limit Definition of the Derivative

Establishing the Derivatives of sin x, cos x & tan x

A Non-Standard Trig Equation | cos(x)=2

PreCalculus - Trigonometry: Trig Identities (31 of 57) Proof cos^2(x)=(1+cos2x)/2

Trigonometry Proof: cos (x + y) = cos x cos y – sin x sin y (https://youtu.be/b0o_dvFkYbU)

Why does sin(2x) = 2sin(x)cos(x)

sin(x)+cos(x)=sqrt(2) | Trigonometry

Why Cos minus theta is cos theta | Cos(-x) barabar | Cos(-x)=cos x proof | Sin(-x) is equal to |

Limit of (1-cos(x))/x as x approaches 0 | Derivative rules | AP Calculus AB | Khan Academy

why sin(2x)=2sin(x)cos(x)

An Interesting Trigonometric Equation | cos(x)=i