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MISCELLANEOUS EXERCISE 4 (II) · Q114

Q.Prove the following by using method of induction: 52n−22n5^{2n} - 2^{2n} is divisible by 33, for all n∈Nn \in N.

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Let P(n):52n−22n=3mP(n):5^{2n}-2^{2n}=3m. Base: n=1n=1: 25−4=21=3(7)25-4=21=3(7); holds. Hypothesis: assume 52k−22k=3m5^{2k}-2^{2k}=3m, i.e. 52k=22k+3m5^{2k}=2^{2k}+3m. Step: $5^{2k+2}-2^{2k+2}=25\cdot5^{2k}-4\cdot2^{2k}=25(2^{2k}+3m)-4\cdot2^{2k}=25\cdot2^{2k}+75m-4\cdot2^{2k}=21\cdot2^{2k}+75m=3(7\cdot2^{2k}+25m) …

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