A positive real number is given. In each move, we can either add 3 to it, subtract 3 to it, multiply it by 3 and divide it by 3 . The sum of all numbers such that after exactly 3 moves, the original number comes back is b a , where a and b are relatively prime positive integers. Find a + b .
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I missed 8 9 ... Darn.
N o t e t h a t u s i n g × o r ÷ a n d − o r + g i v e s s a m e r e s u l t . J u s t o n e i l l u s t r a t i o n b e l o w . 3 x − 3 − 3 = x i s t h e s a m e a s 3 x + 3 + 3 = x . I n b o t h c a s e x = 3 . S o a n o p e r a t i o n t y p e A w i l l u s e × 3 o p e r a t o r . a n d a n o p e r a t i o n t y p e B w i l l u s e − 3 o p e r a t o r . A l s o n o t e t h a t a l l t h r e e o p t i o n s o f t h e s a m e t y p e w i l l n o t g i v e v a l i d s o l u t i o n . S o t h e t h r e e o p e r a t i o n s c a n b e i n a n y o f t h e t h r e e o r d e r g i v e n b e l o w . F i r s t t w o o p e r a t i o n s s a m e t y p e . L a s t t w o s a m e t y p e . F i r s t a n d l a s t s a m e t y p e . S i n c e t h e r e a r e t w o t y p e o f o p e r a t i o n s , A a n d B , w e s h a l l h a v e S I X s o l u t i o n s . A − A − B : − 3 x ∗ 3 − 3 = x , ⟹ 9 x − 3 = x . ∴ x = 8 3 . B − B − A : − ( x − 3 − 3 ) ∗ 3 = x , ⟹ 3 x − 1 8 = x . ∴ x = 9 A − B − B : − 3 x − 3 − 3 = x , ⟹ 3 x − 6 = x . ∴ x = 3 B − A − A : − ( x − 3 ) ∗ 3 ∗ 3 = x , ⟹ 9 x − 2 7 = x . ∴ x = 8 2 7 . A − B − A : − ( 3 x − 3 ) ∗ 3 = x , ⟹ 8 x − 9 = x . ∴ x = 8 9 . B − A − B : − ( x − 3 ) ∗ 3 − 3 = x , ⟹ 3 x − 1 2 = x . ∴ x = 6 S u m o f x v a l u e s ( i n b l u e ) , = 8 1 8 3 = b a . a + b = 1 9 1 .
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∙ The operation is for 3 steps, each step has 4 choices. Thus you will seemingly have 6 4 different sets of operations.
∙ But if your consecutive 2 operations were reverses of each other, then in their case only 1 step should give the same number and this thing is there only for number 0 and the 1 step to be × or ÷
∙ This will happen for following pairs, of consecutive functions " + and − " , " × and ÷ ". This gives 1 6 triples of functions in which only 1 step answer arrives, the answer will only be 0.
∙ In the above cases, if the other (1st or 3rd) operation is + or − , along with the opposites, then there are no solutions, reduces 16 cases, leaving 32 cases to look at.
If there is only + and × , then that will give non-positive solutions , thus remove the 8 additional cases.
When you have a set of the type × and ÷ at the ends, due to the middle function, either answer is 0 or you get inconsistent equation, removes 4 cases.
∙ If the three operations are containing only × and ÷ then answer is 0, there are 8 cases, removing few more !
∙ Computing the other things (i listed most of them, but did that somewhat smartly), just ignore cases that give negatives and you are left with a set of values
3 , 6 , 9 , 8 3 , 8 9 , 8 2 7 .
Their sum is 1 8 + 8 3 9 = 8 1 8 3
Hence answer 1 9 1