if a1=0.4 &r=-2 find a15.

if a1=-10 & d=0.8 find s20

Question

if a1=0.4 &r=-2 find a15.

if a1=-10 & d=0.8 find s20

amistre64 · Answer

first one reads as geometric; the second reads as arithmetic

anonymous · Answer

which formula to use cause I got d instead of r on the first one

amistre64 · Answer

r tends to mean ratio

an=(a1)r^(n-1)

amistre64 · Answer

a15 = .4 * (-2)^14

amistre64 · Answer

s20 indicates the sum of all the numbers from a1 to a20 in the second case

anonymous · Answer

can you check my answer i got -48

amistre64 · Answer

an=a1+d(n-1)  is the arithmetic series

amistre64 · Answer

-2 to an even power is positive

anonymous · Answer

so whats the formulae please

amistre64 · Answer

youve asked 2 questions; which one are you working on?

anonymous · Answer

the first one at the moment

amistre64 · Answer

ok, since r^0 = 1; and we start with n=1 we need to subtract it down to 0

$$a_n=a_1*r^{n-1}$$

anonymous · Answer

thanks and for the second please

amistre64 · Answer

the second one is a bit trickier since they want the sum of the first 20 numbers

amistre64 · Answer

d(0) = 0 so n=1 needs to subtract down to 0
$$a_n=a_1+d(n-1)$$

amistre64 · Answer

the sum of it tho is:
$$S_n=\frac{a_1(a_1+d(n-1))}{n}$$

amistre64 · Answer

thats spoose to be /2

anonymous · Answer

going back to the first one I got a guffy answer can you check it for me

amistre64 · Answer

first*last/2
$$S_n=\frac{a_1(a_1+d(n-1))}{2}$$

anonymous · Answer

3276.8 is that correct

amistre64 · Answer

$$.4*(-2)^{14}=6553.6$$

anonymous · Answer

ok got it thanks

amistre64 · Answer

a1=-10 & d=0.8 find s20

$$S_{20}=\frac{-10(-10+.8(19))}{2}$$

amistre64 · Answer

i get -26 for the second one :)

anonymous · Answer

ok i will check it

anonymous · Answer

I still get -48