Let the first term of the arithmetic progression be $a$ and the common difference be $d$.
Thus:
$X_n = a + (n - 1)d$
From the given conditions:
- $X_5 = a + 4d = -4$
- $2X_6 + 2X_9 = X_{11} + X_{13}$
Using the formula for terms:
- $X_6 = a + 5d$ - $X_9 = a + 8d$ - $X_{11} = a + 10d$ - $X_{13} = a + 12d$
Substitute into the equation:
$2(a + 5d) + 2(a + 8d) = (a + 10d) + (a + 12d)$
Simplifying:
$2a + 10d + 2a + 16d = 2a + 22d$
$4a + 26d = 2a + 22d$
$2a + 4d = 0 \implies a = -2d$
Substitute $a = -2d$ into $X_5 = -4$:
$-2d + 4d = -4 \implies 2d = -4 \implies d = -2$
Now, find $X_{100}$:
$X_{100} = a + 99d = -2(-2) + 99(-2) = 4 - 198 = -194$
Consider an A.P. $a_1,a_2,\ldots,a_n$; $a_1>0$. If $a_2-a_1=-\dfrac{3}{4}$, $a_n=\dfrac{1}{4}a_1$, and \[ \sum_{i=1}^{n} a_i=\frac{525}{2}, \] then $\sum_{i=1}^{17} a_i$ is equal to
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: