Question

giúp mik vs các cậu ơi mik sắp phải nộp r

help me

Tính B=1.2.3+2.3.4+...+(n-1)n(n+1)

cảm ơn mn

Answer 1

$B=\mathrm{1.2.3}+\mathrm{2.3.4}+.........+\left(n-1\right)n\left(n+1\right)$

$\iff 4B=\mathrm{1.2.3.4}+\mathrm{2.3.4.4}+........+\left(n-1\right)n\left(n+1\right).4$

$\iff 4B=\left(4-0\right).1.2.3+\left(5-1\right).2.3.4+.........+\left[\left(n+2\right)-\left(n-2\right)\right]\left(n-1\right)n\left(n+1\right)$

$\iff 4B=\mathrm{1.2.3.4}-\mathrm{0.1.2.3}+\mathrm{2.3.4.5}-\mathrm{1.2.3.4}+.......+\left(n-1\right)n\left(n+1\right)\left(n+2\right)\left(n+3\right)-\left(n-2\right)\left(n-1\right)n\left(n+1\right)$

$\iff 4B=\left(n-1\right)n\left(n+1\right)\left(n+2\right)$

$\iff B=\frac{\left(n-1\right)n\left(n+1\right)\left(n+2\right)}{4}$

Answer 2

Băng Băng 2k6 giúp mik lm ik, mik bận