Question

3.6 + 4.7 + 5.8 +.....(n - 2) terms=​

Answer 1

Answer:

3.6 + 4.7 + 5.8 +.....(n - 2) terms=

the answer is. -45

Answer 2

Answer:

B

Step-by-step explanation:

3.6 + 4.7 + 5.8 +.....(n - 2) = $\sum_{1}^{n-2} (n+2)(n+5)$

= $\sum_{1}^{n-2} n^2+7n+10$

= $\sum_{1}^{n-2} n^2+\sum_{1}^{n-2} 7n+\sum_{1}^{n-2} 10$

= $\frac{(n-2)(n-2+1)(2(n-2)+1)}{6} +7\times \frac{(n-2)(n-2+1)}{2} + 10(n-2)$

= $\frac{(n-2)(n-2+1)(2(n-2)+1)+7\times3\times(n-2)(n-2+1)+10(n-2)\times 6}{6}$

= $\frac{(n-2)(n-1)(2n-3)+21(n-2)(n-1)+60(n-2)}{6}$

= $\frac{(n^2-3n+2)(2n-3)+21(n^2-3n+2)+60n-120}{6}$

=$\frac{2n^3-9n^2+13n-6+21n^2-63n+42+60n-120}{6}$

= $\frac{2n^3+12n^2+10n-84}{6}$