Question

what is the sum of first 10 terms of the a.p 15,10 5​

Answer 1

Answer:

105

Step-by-step explanation:

a= 15, d= 10-15 =-5 n=10...

Sn = n/2 [2a + ( n-1) ×d]

= 10/2 [2×15+(10-1)×(-5)]

= 5[30+9×(-5)]

=5×30-45

=105

Answer 2

Answer:-

the sum of first 10 terms of the A.P. 15, 10, 5 is 375.

Solution:-

The given A.P. is - 15, 10, 5...

Here,

The first term (a) = 15

The difference between the consecutive terms (d) = 15 - 10 = 10 - 5 = 5

$S_n = \frac{n}{2} (2a + (n - 1)d)$

$S_n = Sum \: \: of \: \: the \: \: n \: \: terms$

In the question, it is given to find the sum of first 10 terms that means n = 10

$S_{10}= \frac{10}{2} (2 \times 15 + (10 - 1)5) \\ \\ S_{10}= \frac{10}{2} (2 \times 15 + (9)5) \\ \\ S_{10}= \frac{10}{2} (30 + 45) \\ \\ S_{10}= \frac{10}{2} \times 75 \\ \\ S_{10}= 5\times 75 \\ \\ S_{10}= 375$

Therefore, the sum of first 10 terms of the A.P. 15, 10, 5 is 375.