Question

Tính bằng cách hợp lí : \(\dfrac{15^3+5.15^2-5^3}{18^3+6.18^2-6^3}\)

Answer 1

\(\dfrac{15^3+5.15^2-5^3}{18^3+6.18^2-6^3}\)

= \(\dfrac{\left(5.3\right)^3+5.\left(5.3\right)^2-5^3}{\left(6.3\right)^3+6.\left(6.3\right)^2-6^3}\)

= \(\dfrac{5^3.3^3+5^3.3^2-5^3}{6^3.3^3+6^3.3^2-6^3}\)

= \(\dfrac{5^3.\left(3^3+3^2-1\right)}{6^3.\left(3^3+3^2-1\right)}\)

=\(\dfrac{5^3}{6^3}=\dfrac{125}{216}\)

Answer 2