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+---
+epic: 6
+story: 6.1
+title: "Trigonometric Functions"
+status: draft
+---
+
+## Epic 6 — Advanced Math & Functions
+**Goal:** Scientific, financial, and power-user math.
+
+### Story 6.1: Trigonometric Functions
+
+As a **CalcPad user**,
+I want to evaluate trigonometric functions including sin, cos, tan, and their inverses and hyperbolic variants,
+So that I can perform scientific and engineering calculations directly in CalcPad.
+
+**Acceptance Criteria:**
+
+**Given** the angle mode is set to radians (default)
+**When** the user writes `sin(3.14159)` (approximately pi)
+**Then** the result is approximately `0` (within floating-point tolerance)
+
+**Given** the angle mode is set to degrees
+**When** the user writes `sin(90)`
+**Then** the result is `1`
+
+**Given** the user writes `sin(45°)` with the degree symbol
+**When** the engine evaluates the expression
+**Then** the engine treats the argument as degrees regardless of the global angle mode setting
+**And** the result is approximately `0.7071`
+
+**Given** the engine is in radians mode
+**When** the user writes `cos(0)`
+**Then** the result is `1`
+
+**Given** the user writes `tan(45°)`
+**When** the engine evaluates
+**Then** the result is approximately `1`
+
+**Given** the user writes `asin(1)` in radians mode
+**When** the engine evaluates
+**Then** the result is approximately `1.5708` (pi/2)
+
+**Given** the user writes `acos(0.5)` in degrees mode
+**When** the engine evaluates
+**Then** the result is `60` (degrees)
+
+**Given** the user writes `atan(1)` in degrees mode
+**When** the engine evaluates
+**Then** the result is `45`
+
+**Given** the user writes `sinh(1)`
+**When** the engine evaluates
+**Then** the result is approximately `1.1752`
+
+**Given** the user writes `cosh(0)`
+**When** the engine evaluates
+**Then** the result is `1`
+
+**Given** the user writes `tanh(0)`
+**When** the engine evaluates
+**Then** the result is `0`
+
+**Given** the user writes `sin(x)` where `x` is out of domain for an inverse trig function (e.g., `asin(2)`)
+**When** the engine evaluates
+**Then** an error is displayed indicating the argument is out of the valid domain
+
+**Given** the user wants to switch angle modes
+**When** the user sets the mode via a configuration directive (e.g., `angle mode: degrees`)
+**Then** all subsequent trig evaluations use the selected mode until changed again