Three little integers

Find the value of a + b + c a+b+c , where a , b a, b and c c are non-negativ integers, such that 3 a + 17 × 4 b = c 2 . 3^a+17\times 4^b=c^2 .


The answer is 36.

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1 solution

Step 1: By observation, 17 × 4 b 17\times 4^b when divided by 3 3 will always remain 2 2 .

The only way to make c 2 c^2 possible is to make 3 a = 1 3^a = 1 .

a = 0 a = 0

Step 2: That makes c = 3 n c = 3n .

1 + 17 × 4 b = 9 n 2 \because 1 + 17\times 4^b = 9n^2 ( 3 n ) 2 = 9 n 2 (3n)^2 = 9n^2

17 × 4 b 8 ( m o d 9 ) 17\times 4^b \equiv 8 \pmod 9

Step 3: We will have to make 17 × 2 x 17\times 2^x and 2 y 2^y differ by 2.

The only solution set is ( 32 , 34 ) (32,34) , which is 17 × 2 1 17\times 2^1 and 2 5 2^5 . 2 1 × 2 5 = 4 3 2^1\times 2^5 = 4^3

b = 3 b=3

Step 4: Conclude out that c = 33 c=33

a + b + c = 36 a+b+c = 36

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