Asequence has an initial value of 10 and each term is twice the previous term.which function models this sequence

1.   Using the formula for nth term ( = a1 * r^(n-1)

the 5th term = (-2)*(1/4)^4

= -2/256  = -1/128  (answer)

2.   The 4 terms between 1 and  32 are found as follows:-

a1 = 1 and  a6 = 32   so  32/1 = a6/a1

 therefore 32/1 = a1r^5 / a1

so r^5 = 32 and r = 2

so 4 terms are a1r,  a1r^2, a1^r^3 and a1r^4

= 2, 4, 8, 16   (answer)

3.   You need the sum of 10 terms where a1 = -2 and r = 3

S10  = -2 * (3^10 - 1) / (3 - 1)

= -59058

4.  Sum to infinity  = a1 / (1 - r)

a1 = 1 and r = 1/2

so the answer is  1 /  (1 - 1/2)

= 1 / 1/2

= 2   (answer)