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# Math Quiz: 9 is number one

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Getting "follow" information
• when

a = b

multiply both sides by 8a

8a^2 = 8ab

add (a^2 - 9ab) to both sides

8a^2 + (a^2 - 9ab) = 8ab + (a^2 - 9ab)

simplify them to

9(a^2 - ab) = (a^2 - ab)

divide both sides by (a^2 - ab)

9 = 1

• There's a a^2 - ab ≠ 0 missing somewhere, I think

(by the way, I suck at math)

• bureX said:

There's a a^2 - ab ≠ 0 missing somewhere, I think

(by the way, I suck at math)

That's right.

9(a^2 - ab) = (a^2 - ab)  → 0 = 0

• algorith said:
bureX said:
*snip*

That's right.

9(a^2 - ab) = (a^2 - ab)  → 0 = 0

OK!
Another one,

lim x->0 [{sin(9x)}/x] = ?

• bureX said:

There's a a^2 - ab ≠ 0 missing somewhere, I think

(by the way, I suck at math)

if a == b, then a^2 == ab and a^2-ab==0. Division by zero is not permitted here  You can get any answer you want if you divide by zero, just as you can "prove" any proposition if you accept a contradiction, just as any statement about members of the empty set is logically true!

• brianbec said:
bureX said:
*snip*

if a == b, then a^2 == ab and a^2-ab==0. Division by zero is not permitted here  You can get any answer you want if you divide by zero, just as you can "prove" any proposition if you accept a contradiction, just as any statement about members of the empty set is logically true!

This statement is false.

• iStation said:
algorith said:
*snip*

OK!
Another one,

lim x->0 [{sin(9x)}/x] = ?

Nice one. Here's another

Find the first positive real value of t for which the following is zero

256 cos^9(pi t / 162) - 576 cos^7(pi t / 162) + 432 cos^5(pi t / 162) -120 cos^3(pi t/162) + 9 cos(pi t / 162)

• brianbec said:
iStation said:
*snip*

Nice one. Here's another

Find the first positive real value of t for which the following is zero

256 cos^9(pi t / 162) - 576 cos^7(pi t / 162) + 432 cos^5(pi t / 162) -120 cos^3(pi t/162) + 9 cos(pi t / 162)

Wow!

9! Right? [27, 45, 63, 81, 99....]

+ ∞
∫ [{sin(9x)}/x]dx = ?
− ∞

• @iStation:

You are dividing by 0!