a+2b=5 show that a+8b cube+30ab=125
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since
(a+2b)=5,
then
(a+2b)³=5³,
a³+8b³+3×a×2b(a+2b)=125,
a³+8b³+6ab×5=125,
then
a³+8b³+30ab=125
(a+2b)=5,
then
(a+2b)³=5³,
a³+8b³+3×a×2b(a+2b)=125,
a³+8b³+6ab×5=125,
then
a³+8b³+30ab=125
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