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A physical quantity P is described by the relation P=a^1/2 b² c³ d⁻⁴. If the relative errors in the measurement of a, b, c and d respectively, are 2%, 1%, 3% and 5%, then the relative error in P will be

Asked in JEE Main Online 2017 · Error in a power

Answer: (4) 32%

Step-by-step solution

Idea: every exponent weights its error in magnitude; the negative exponent on d still adds.

(Δ P)/P=1/2(Δ a)/a+2(Δ b)/b+3(Δ c)/c+4(Δ d)/d

=1/2(2%)+2(1%)+3(3%)+4(5%)

=1%+2%+9%+20%=32%.

The printed key gives 8%, which no reading of the exponents produces; the derivation stands. See DEFECTS.

Why the other options are wrong

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