# Calculate the radius of Eris using equation 21.1, where R = radius in km,

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Calculate the radius of Eris using equation 21.1, where R = radius in km, r = heliocentric distance (97.6 AU), ) = distance from the Earth (96.6 AU), p = albedo expressed as a decimal fraction (0.86), and V = visual magnitude (18.8).

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R2 = (4.53 x 105r2Δ2)/(100.4v x p)

0.4 x 18.8 = 7.52

107.52 = 33.113 x 106

R2 = ((453 x 105 x (97.6)2(96.6)2)/(33.113 x 106 x 0.86)

R2 = 14.140 x 106 x 10-1

R = (14.140 x 105)1/2 = 1189.0km

Note: The radius of Pluto is 1150 km (i.e., Eris is larger than Pluto).