To sum it is to damp it

Algebra Level 3

I f a 2 + b 3 + c 6 = 2009 + 918 2 + 272 3 + 72 6 a n d a , b , c Z + f i n d t h e v a l u e o f a + b + c If\quad a\sqrt { 2 } +b\sqrt { 3 } +c\sqrt { 6 } =\sqrt { 2009+918\sqrt { 2 } +272\sqrt { 3 } +72\sqrt { 6 } } \\and\quad a,b,c\quad \in \quad { ℤ }^{ + }\quad \\ find\quad the\quad value\quad of\quad a+b+c

26 18 42 30

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2 solutions

Kay Xspre
Oct 6, 2015

Square both sides will gives 2 a 2 + 3 b 2 + 6 c 2 + 2 a b 6 + 4 a c 3 + 6 b c 2 = 2009 + 918 2 + 272 3 + 72 6 2a^2+3b^2+6c^2+2ab\sqrt{6}+4ac\sqrt{3}+6bc\sqrt{2} = 2009+918\sqrt{2}+272\sqrt{3}+72\sqrt{6} By observing the coefficient a b = 36 ab = 36 , a c = 68 ac = 68 , b c = 153 bc = 153 . Multiply them all to ( a b c ) 2 = ( 6 × 2 × 3 × 17 ) 2 (abc)^2 = (6\times2\times3\times17)^2 . As all variables are positive, then a b c = 612 abc = 612 . Solve for each gives ( a , b , c ) = ( 4 , 9 , 17 ) (a, b, c) = (4, 9, 17) so a + b + c = 4 + 9 + 17 = 30 a+b+c = 4+9+17 = 30

Tai Ching Kan
Oct 29, 2015

Squaring both sides,

2 a 2 + 3 b 2 + 6 c 2 + 2 a b 6 + 2 a c 12 + 2 b c 18 = 2009 + 918 2 + 272 3 + 72 6 2a^{2}+3b^{2}+6c^{2}+2ab\sqrt{6}+2ac\sqrt{12}+2bc\sqrt{18}=2009+918\sqrt{2}+272\sqrt{3}+72\sqrt{6}

2 a 2 + 3 b 2 + 6 c 2 + 2 a b 6 + 4 a c 3 + 6 b c 2 = 2009 + 918 2 + 272 3 + 72 6 2a^{2}+3b^{2}+6c^{2}+2ab\sqrt{6}+4ac\sqrt{3}+6bc\sqrt{2}=2009+918\sqrt{2}+272\sqrt{3}+72\sqrt{6}

Since a a , b b and c c are positive integers,

2 a 2 + 3 b 2 + 6 c 2 = 2009 2a^{2}+3b^{2}+6c^{2}=2009 ( ) (*)

2 a b 6 = 72 6 2ab\sqrt{6}=72\sqrt{6}

4 a c 3 = 272 3 4ac\sqrt{3}=272\sqrt{3}

6 b c 2 = 918 2 6bc\sqrt{2}=918\sqrt{2}

a b = 36 = 4 × 9 ab=36=4\times9

a c = 68 = 4 × 17 ac=68=4\times17

b c = 153 = 9 × 17 bc=153=9\times17

a = 4 a=4 , b = 9 b=9 , c = 17 c=17

Checking our solution against ( ) (*) ,

2 a 2 + 3 b 2 + 6 c 2 = 2 × 16 + 3 × 81 + 6 × 289 = 2009 2a^{2}+3b^{2}+6c^{2}=2\times16+3\times81+6\times289=2009

Therefore,

a + b + c = 30 a+b+c=\boxed{30}

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