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Question 8
Let A be the largest positive integer that divides all the numbers of the form $$3^k + 4^k + 5^k$$, and B be the largest positive integer that divides all the numbers of the form $$4^k + 3(4^k) + 4^{k + 2}$$ , where k is any positive integer. Then (A + B) equals
Correct Answer: 82
Solution
A is the HCF of $$3^k+4^k+5^k$$ for different values of k.
For k = 1, value is 12
For k = 2, value is 50
For k = 3, value is 216
HCF is 2. Therefore, A = 2
$$4^k+3\left(4^k\right)+4^{k+2}=4^k\left(1+3\right)+4^{k+2}=4^{k+1}+4^{k+2}=4^{k+1}\left(1+4\right)=5\cdot4^{k+1}$$
HCF of the values is when k = 1, i.e. 5*16 = 80
Therefore, B = 80
A + B = 82
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