Question

The sum of the series 3.6 + 4.7 + 5.8 + ........ upto (n - 2) terms

• A.
• B.
• C.
• D. None of these

Answer 1

The correct option is B

S = 3.6 + 4.7 + ........ upto n - 2 terms

= (1.4 + 2.5 + 3.6 + 4.7 + .........upto n terms) - 14

= $\sum$n(n + 3) - 14 = $\frac{1}{6}$(2 $n^3$ + 12 $n^2$ + 10n) - 14

= ( $\frac{2n^3+12n^2+10n-84}{6}$), where n = 3, 4, 5.......

Trick : $S_1$ = 18, $S_2$ = 46

Now put in options (n - 2) = 1, 2 i.e. n = 3, 4

Obviously (b) gives the values.