Power negative means concave lens. f = 1/P (in metre) = 1/(-2.5) = -0.4 m = -40 cm. So concave lens of focal length 40 cm. Focal length negative for concave.
Explanation:
Fuse melts and breaks circuit when current exceeds rating. MCB (Miniature Circuit Breaker) does same but resettable. Switch only turns on/off. Resistor limits current but doesn't break.
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