Question

if a+2b =5 ; then show that : a^3+8b^3+30ab=125

Answer 1

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good evening !!

a+2b = 5
a= 5-2b

then take left side part from equation :

a cube + 8b cube + 30 ab
= (5-2b) cube + 8b cube + 30 (5-2b)b
= 125 - 8b cube + 3(5)(-2b)(5-2b) + 8b cube + 30 (5-2b)b

= 125 -30 b ( 5-2b) + 30(5-2b)b + 8b cube - 8b cube

= 125

then left side part = right side part


proove that ....