Question

Find a15 and s15 if an = 9-5n

Answer 1

ANSWER

a15 = 9-5(15)=9-75=-66

a1=9-5(1)=4

a2=9-5(2)=-1

So Common difference "d"= a2-a1=-1-4=-5

$s15 = \frac{15}{2} (2 \times 4 + (15 - 1) \times - 5) \\ = \frac{15}{2} (8 + ( - 70)) \\ = \frac{15}{2} ( - 62) \\ = 15 \times - 31 \\ = - 465$

which is the required ans