Question

find the co efficient of a³ in (2a-5)³

Answer 1

When (2a-5)^3 will be expanded, the only term that will give us a^3 will be (2a)^3 which is = 8a^3. Hence the coeffecient of a^3 will be 8.
[ Expansion of (2a-5)^3 => (2a)^3 - 5^3 - 3 (2a)^2×5 + 3×2a×5^2]

Answer 2

(2a - 5 )^3
We know that ( a - b ) ^3 = a^3 - b^3 + 3ab^2 - 3a^2b
=> 2a^3 - 5^3 - 3 × (2a)^2×5 + 3 × 2a × 5^2
=> 8a^3 - 125 - 60a^2 + 150a
We can see that coefficient of a^3 is 8.