Help I don’t get how to do this question I know the sum to infinity formula

It’s in my forums booklet

A/1-r

But idk how to do this

It’s in my forums booklet

A/1-r

But idk how to do this

(edited 12 months ago)

Is the answer actually 6? because would multiplying each term mean the new sequence's first term is -2a (if a is the first term of the original sequence) and the common ratio is still the same as before. That would mean the new sum is -2a/(1-r) = -2(-3) which is 6.

But I'm not 100% sure of this method

But I'm not 100% sure of this method

I've done this question before and yes it is 6. I think your method is the way you do it

(edited 12 months ago)

[sum to infinity] = [term 1] + [term 2] + [term 3] + [term 4] + ...

if you multiply each term by -2 then you get [new sum to infinity] = (-2) * [term 1] + (-2) * [term 2] + (-2) * [term 3] + (-2) * [term 4] + ...

therefore [new sum to infinity] = (-2) * ( [term 1] + [term 2] + [term 3] + [term 4] + ... ) = (-2) * [old sum to infinity]

so as the [old sum to infinity] = -3

[new sum to infinity] = (-2) * (-3) = 6

if you multiply each term by -2 then you get [new sum to infinity] = (-2) * [term 1] + (-2) * [term 2] + (-2) * [term 3] + (-2) * [term 4] + ...

therefore [new sum to infinity] = (-2) * ( [term 1] + [term 2] + [term 3] + [term 4] + ... ) = (-2) * [old sum to infinity]

so as the [old sum to infinity] = -3

[new sum to infinity] = (-2) * (-3) = 6

Original post by Marvel_fan

Is the answer actually 6? because would multiplying each term mean the new sequence's first term is -2a (if a is the first term of the original sequence) and the common ratio is still the same as before. That would mean the new sum is -2a/(1-r) = -2(-3) which is 6.

But I'm not 100% sure of this method

But I'm not 100% sure of this method

Yea the answer is 6 but idk how 😭 have u got your maths a level this year??

I don't know if my method is the best but I just thought that if every term had been multiplied by -2, then the sum to infinity would be times by -2 aswell, which means the answer would be 6. This is the same as what @_Hello_There_ has written i've just realised

Original post by Alevelhelp.1

Yea the answer is 6 but idk how 😭 have u got your maths a level this year??

Yeah I have! have you as well?

First sequence: a + ar + ar^2 + ar^3 + ...

Second sequence: -2a + -2ar + -2ar^2 + -2ar^3 + ... = -2a + (-2a)r + -(2a)r^2 + (-2a)r^3 + ...

Not sure if that helps at all but its how I visualize it

(edited 12 months ago)

Original post by Marvel_fan

Yeah I have! have you as well?

First sequence: a + ar + ar^2 + ar^3 + ...

Second sequence: -2a + -2ar + -2ar^2 + -2ar^3 + ... = -2a + (-2a)r + -(2a)r^2 + (-2a)r^3 + ...

Not sure if that helps at all but its how I visualize it

First sequence: a + ar + ar^2 + ar^3 + ...

Second sequence: -2a + -2ar + -2ar^2 + -2ar^3 + ... = -2a + (-2a)r + -(2a)r^2 + (-2a)r^3 + ...

Not sure if that helps at all but its how I visualize it

Yesss mine is on Tuesday my pure test I’m doing aqa btw wbu I’m so scared I’m gonna flop… I’ve been getting LOW Cs 😭😭😭

plz pray for me 🤲

(edited 12 months ago)

Original post by Alevelhelp.1

Yesss mine is on Tuesday my pure test I’m doing aqa btw wbu I’m so scared I’m gonna flop… I’ve been getting LOW Cs 😭😭😭

plz pray for me 🤲

plz pray for me 🤲

No you've got this!! I do edexcel but have further on monday too. Just keep doing your practice questions and I'm sure you'll do great

Original sum = -3.

New sum = -2 * Original sum = -2 * -3 = 6.

One does not need to deal with the series terms at all in this case.

New sum = -2 * Original sum = -2 * -3 = 6.

One does not need to deal with the series terms at all in this case.

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