Question

2+5+8+11+......+x, Sn=155 find x. Please explain with solution.

Answer 1

a=2
d=3
$155 = \frac{n}{2} (2 \times 2 + (n - 1)3) \\ 155 \times 2 = n(4 + 3n - 3) \\ 310 = n + 3n {}^{2} \\ 3 {n}^{2} + n - 310 = 0$
by solving the quadratic equation
the value of n comes 10.
tn = 10
xth term is 10th term
x or t10 = a + 9d
2+ 9 × 3
2+27
29
therefore the value of x is 29.
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