Ratio of Squares of Integers of Opposite Signs

CAT 2017 Slot 1 · QA · Hard · Algebra

If aa and bb are integers of opposite signs such that (a+3)2:b2=9:1(a + 3)^2 : b^2 = 9 : 1 and (a1)2:(b1)2=4:1(a - 1)^2 : (b - 1)^2 = 4 : 1, then the ratio a2:b2a^2 : b^2 is:

  1. A.

    9 : 4

  2. B.

    81 : 4

  3. C.

    1 : 4

  4. D.

    25 : 4

Answer

D

Explanation

From (a+3)2/b2=9    a+3=±3b(a + 3)^2 / b^2 = 9 \implies a + 3 = \pm 3b. From (a1)2/(b1)2=4    a1=±2(b1)(a - 1)^2 / (b - 1)^2 = 4 \implies a - 1 = \pm 2(b - 1). Since a,ba, b are integers of opposite signs, test a+3=3b    a=3b3a + 3 = -3b \implies a = -3b - 3. Substitute into a1=2(b1)    3b4=2b2    5b=2a - 1 = 2(b - 1) \implies -3b - 4 = 2b - 2 \implies 5b = -2 (not integer). Test a1=2(b1)=2b+2    3b4=2b+2    b=6    a=15a - 1 = -2(b - 1) = -2b + 2 \implies -3b - 4 = -2b + 2 \implies b = -6 \implies a = 15. Opposite signs check: a=15,b=6a = 15, b = -6 are opposite signs. (a+3)2:b2=182:(6)2=324:36=9:1(a+3)^2 : b^2 = 18^2 : (-6)^2 = 324 : 36 = 9 : 1. (a1)2:(b1)2=142:(7)2=196:49=4:1(a-1)^2 : (b-1)^2 = 14^2 : (-7)^2 = 196 : 49 = 4 : 1. a2:b2=152:(6)2=225:36=25:4a^2 : b^2 = 15^2 : (-6)^2 = 225 : 36 = 25 : 4.

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