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7. Given an arithmetic sequence, find a26if a4 = 71 and a32 = 1. 8. Given a geometric sequence, find az ifa1 = 729 and a2 = – 243.

Sagot :

Arithmetic sequence

a= 26

a4 = 71

a32 = 1.

an = a1 + (n-1)d

a4 = a1 + (3)*d

71 = a1 + (3)*d (I)

a32 = a1 + (31)*d

1 = a1 + 31 d (II)

to find a1 and d we subtract (II)- (I)

1 = a1 + 31 d (II)

-( 71 = a1 + (3)*d) (I)

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-70 = 0 28 d

-70 = 28 d

d= -70/ 28

d= -5/2

Replacing d in (I)

71 = a1 - (3)(5/2)

a1= 71 + (3)(5/2)

a1= 78.5

The arithmetic sequence is

an = a1 + (n-1)d

an = 78.5 + 2.5 (n-1)

Verifying

a4 = 78.5 - 2.5 (3) = 78.5 -7.5= 71

a32 = 78.5 +2.5 (31) =78.5 -77.5 = 1

a26 = 78.5 + 2.5 (25) = 78.5 - 62.5 = 16

Give me some minutes

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Answer

a26 = 16