hi! I’m an IB student who is doing homework on geometric series, but this one is very hard!

[oblique beacon]

It’s really hard 😭😭😭

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[waxen meteor]

uh

zoom in?

@oblique beacon

Oh use the formula for the sum of a geometric ratio to find r

[oblique beacon]

I tried that

I got r=-1

Which is impossible

It’s meant to be done in pairs but we can’t crack it

[waxen meteor]

81,3?

[oblique beacon]

81.3

But we’re meant to find the common ratio and u1

81.3 is the sum of the first three terms

[waxen meteor]

$\frac{a_1(1-r^3)}{1-r} = 81.3$

$\frac{a_1}{1-r} = 300$

[pliant rivet]

wolfqz

[waxen meteor]

solve for r here

doing by substitution is very easy

[oblique beacon]

Oh I forgot about substitution

[waxen meteor]

lol

[oblique beacon]

I spent 45 min of my life for nothing 😫

[waxen meteor]

dont worry

sometimes u miss the easiest methods

[oblique beacon]

Ye

It’s just you don’t normally think of substitution in these situations

Especially at my level

U know?

[waxen meteor]

i didtn either

but when i saw the 1-r goes to the other side