Question
If $$7^9 + 9^7$$ is divided by 64 then the remainder is
A.
0
B.
1
C.
2
D.
63
Answer :
0
Solution :
We have,
$$\eqalign{
& {7^9} + {9^7} = {\left( {8 - 1} \right)^9} + {\left( {8 + 1} \right)^7} = {\left( {1 + 8} \right)^7} - {\left( {1 - 8} \right)^9} \cr
& = \left[ {1 + {\,^7}{C_1} 8 + {\,^7}{C_2} {8^2} + ..... + {\,^7}{C_7} {8^7}} \right] - \left[ {1 - {\,^9}{C_1} 8 + {\,^9}{C_2} {8^2} - ..... - {\,^9}{C_9} {8^9}} \right] \cr
& = {\,^7}{C_1} 8 + {\,^9}{C_1} 8 + \left[ {^7{C_2} + {\,^7}{C_3} 8 + ..... - {\,^9}{C_2} + {\,^9}{C_3} 8 - .....} \right]{8^2} \cr
& = 8\left( {7 + 9} \right) + 64k = 8 \cdot 16 + 64k = 64q, \cr} $$
where $$q = k + 2$$
Thus, $$7^9 + 9^7$$ is divisible by 64.