Đề bài
Tính:
\(\eqalign{ & a)\,\,C = {{{8^{10}} + {4^{10}}} \over {{8^4} + {4^{11}}}} \cr & b)\,\,D = {1 \over {2.3}} + {1 \over {3.4}} + {1 \over {4.5}} + ...... + {1 \over {2015.2016}} \cr & c)\,\,E = {1 \over 3} - {3 \over 4} + {3 \over 5} + {2 \over {2015}} - {1 \over {36}} + {1 \over {15}} - {2 \over 9} \cr} \)
Lời giải chi tiết
\(\eqalign{ & a)C = {{{8^{10}} + {4^{10}}} \over {{8^4} + {4^{11}}}} = {{{{({2^3})}^{10}} + {{({2^2})}^{10}}} \over {{{({2^3})}^4} + {{({2^2})}^{11}}}} = {{{2^{20}}({2^{10}} + 1)} \over {{2^{12}}(1 + {2^{10}})}} = {{{2^{20}}} \over {{2^{12}}}} = {2^8} = 256 \cr & b)D = {1 \over {2.3}} + {1 \over {3.4}} + {1 \over {4.5}} + ... + {1 \over {2015.2016}} \cr & = {1 \over 2} - {1 \over 3} + {1 \over 3} - {1 \over 4} + {1 \over 4} - {1 \over 5} + ... + {1 \over {2015}} - {1 \over {2016}} = {1 \over 2} - {1 \over {2016}} = {{1007} \over {2016}} \cr & c)E = {1 \over 3} - {3 \over 4} + {3 \over 5} + {2 \over {2015}} - {1 \over {36}} + {1 \over {15}} - {2 \over 9} = {1 \over 3} + {3 \over 5} + {1 \over {15}} - {3 \over 4} - {1 \over {36}} - {2 \over 9} + {2 \over {2015}} \cr & = \left( {{5 \over {15}} + {9 \over {15}} + {1 \over {15}}} \right) + \left( {{{ - 27} \over {36}} - {1 \over {36}} - {8 \over {36}}} \right) + {2 \over {2015}} \cr & = {{15} \over {15}} + \left( {{{ - 36} \over {36}}} \right) + {2 \over {2015}} = 1 + ( - 1) + {2 \over {2015}} = {2 \over {2015}} \cr} \)
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