Question

Solve the ques 4 in the attachment

Optional question of NCERT

Class 10

Arithmetic Progressions

Answer 1

Hope you understand it.

Thank you.

Answer 2

Hi there ♥️

Let the first term be 1 and common difference be 1.

【These steps are in picture 】 Please look at the attachment.

According to question,

Sx-1 = S49- Sx

$\frac{ {x}^{2} - x}{2} = 49 \times 25 - \frac{ {x}^{2} + x}{2} \\ \\ \frac{ {x}^{2} - x}{2} + \: \frac{ {x}^{2} +x}{2} = 49 \times 25 \\ \\ \frac{ {x }^{2} - x + {x}^{2} + x }{2} = 49 \times 25 \\ \\ {x}^{2} = 49 \times 25$

x = ± 7 × 5

Since, x is a counting number, so taking positive square root,

x= 7 × 5 = 35

Hope it will helpful :)

Thanks !.