Question

Also find the sum of first 15 terms.​

Answer 1

Answer:

120

Step-by-step explanation:

The sum of the first 15 terms is the sum of the numbers 1-15.

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15

= 120

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Answer 2

Answer:

Substituting n=15,a=10,d=4 in the above equation, we have S15=152[2(10)+(15−1)4]. Simplifying the above expression, we have S15=570. Hence, the sum of the first 15 terms of an AP whose fifth and ninth terms are 26 and 42 respectively is given by 570.

I'm not sure that the ans. is completely correct..

but it's bit correct..

HOPE ANS. IS HELPFUL..