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C1 Arithmetic sequence help!!

mujahid_e3

Can someone help me answer part c of this question? Question is in attachment!

Thanks in advance!

c1.JPG58.2KB

Reply 1

fudgesundae

Original post by mujahid_e3

Can someone help me answer part c of this question? Question is in attachment!

Thanks in advance!

Use the formula n/2{2A + (n-1)d} = Sum (twice Jill's value)

n = number of terms
A= 1st term
d = common difference

sub in the values you know you get:

20/2{2A + 19 x 30} = 9800
10{2A +570} = 9800
2A +570 = 980
2A = 410
A = 205

Reply 2

ttoby

Use the formula

S_n=\frac{1}{2}n\{2a+(n-1)d\}

.

In this case,

S_n

is double your answer to part (b), a is A, n is 20 and d is 30.

Then you can rearrange to find A.

Reply 3

mujahid_e3

Original post by ttoby

Use the formula

S_n=\frac{1}{2}n\{2a+(n-1)d\}

.

In this case,

S_n

is double your answer to part (b), a is A, n is 20 and d is 30.

Then you can rearrange to find A.

i am finding it hard to rearrange and find a so if you can please show step by step!

Thanks

Reply 4

fudgesundae

Original post by fudgesundae

Use the formula

n = number of terms
A= 1st term
d = common difference

sub in the values you know you get:

20/2{2A + 19 x 30} = 9800
10{2A +570} = 9800
2A +570 = 980
2A = 410
A = 205

^^^^^^^

(edited 13 years ago)