Pattern 1: Sign Convention + Mirror/Lens Formula (Highest Frequency)

Years tested: 2018, 2020, 2023 $\frac{mirror}{lens formula with sign convention}$ Typical question: "Object placed X cm in front of [concave/convex] [mirror/lens] of focal length Y cm. Find image position and nature." Key trap: Wrong sign for f (especially concave mirror with positive f) or using mirror magnification for a lens. Strategy: Step 1 — assign signs (u negative, f negative for concave mirror / positive for convex lens). Step 2 — apply formula. Step 3 — interpret v sign (mirror: negative = real; lens: positive = real).