## anonymous one year ago Solve: 16x2 − 81 = 0. x = 4, 9 x = −4, −9 x = 9 over 4, x = −9 over 4 x = 81 over 16, x = −81 over 16

1. anonymous

@welshfella

2. welshfella

note that 16x^2 and 81 are 2 perfect squares so what are the factors of 16x^2 - 81?

3. welshfella

this is the difference of 2 squares a^2 - b^2 = (a + b)(a - b)

4. anonymous

I do believe the answer is D.

5. welshfella

try to factor the expression compare 16x^2 - 81 with a^2 - b^2 and plug the values in

6. anonymous

16^12-81^2?

7. welshfella

what are the square roots of 16x^2 and 81?

8. welshfella

no a and b are the square roots of 16x^2 and 81

9. anonymous

I think the answer is D. Am i correct?

10. welshfella

no

11. welshfella

what number multiplied by itself = 81?

12. anonymous

The answer I belive it is @welshfella make sure I am correct on this lol. B.

13. anonymous

@welshfella is she correct?

14. welshfella

no its not d the important thing is you learn how to solve this

15. anonymous

ok

16. welshfella

ok lets do it another way 16x^2 - 81 = 0 add 81 to both sides 16x^2 = 81 now divide both sides by 16 what do you get?

17. anonymous

I am confused.

18. welshfella

|dw:1439660364573:dw|

19. anonymous

$\sqrt{81/16}$

20. welshfella

now all you need do is find the 2 square roots of 16/81 - dont forget therea-s a positive and a negative square root

21. welshfella

now what is the square root of 81 and 16 ?

22. anonymous

Um 2.25

23. welshfella

2.25 is one square root but what is the other?

24. anonymous

-2.25

25. welshfella

2.225 is square root of 81/16

26. welshfella

yes but they want the results as a fraction what are they as fractions?

27. welshfella

* 2.25 i meant

28. welshfella

|dw:1439660690023:dw|

29. welshfella

really - she has left!!

30. anonymous

Sorry my internet went out :/

31. anonymous

@welshfella

32. welshfella

ok so what are the square roots of 81 and 16 ?

33. anonymous

9/4?

34. anonymous

yes correct :]

35. welshfella

also -9/4