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Thread: Power at 180 - What am I doing wrong? Help!

  1. #1
    Rising Star Lori's Avatar
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    Power at 180 - What am I doing wrong? Help!

    Possible Brain Fart
    Help!
    I do the calcs but my answer can't be right
    here's the RX
    +1.75-0.50X75

    Here's the math steps I'm using;

    180-75=105

    sin
    squared
    multiplied
    =0.48.......
    + +1.75
    =+2.23 @180
    that can't be right

    what am I missing?
    I'm feeling stupid right about now....

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    Quote Originally Posted by Lori View Post
    Possible Brain Fart
    Help!
    I do the calcs but my answer can't be right
    here's the RX
    +1.75-0.50X75

    Here's the math steps I'm using;

    180-75=105

    sin
    squared
    multiplied
    =0.48.......
    + +1.75
    =+2.23 @180
    that can't be right

    what am I missing?
    I'm feeling stupid right about now....
    You need to subtract .48as it has a negative value

  3. #3
    Rising Star Lori's Avatar
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    at what step do i subtract it; subtract it from what? sorry, not a math wiz.

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    Quote Originally Posted by Lensman11 View Post
    You need to subtract .48as it has a negative value
    +1

    In similar problems (plus lens with minus cyl), just realize the answer will never be higher than the plus power.

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    Rising Star Lori's Avatar
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    Quote Originally Posted by Lensman11 View Post
    You need to subtract .48as it has a negative value
    At what step do i subtract it? subtract it from what? sorry, not a math wiz...

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    Quote Originally Posted by jefe View Post
    +1

    In similar problems (plus lens with minus cyl), just realize the answer will never be higher than the plus power.
    Exactly, that's how I knew it is wrong.

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    Quote Originally Posted by Lori View Post
    At what step do i subtract it? subtract it from what? sorry, not a math wiz...
    Subtract from +1.75 --- BUT --- don't say add or subtract, say combine. Combine two plus numbers, get a bigger number. Combine two minus numbers, get a bigger number (it's minus, but still bigger). Combine a plus and a minus (like your example) and get a smaller number. Will it be plus or minus? Depends on which was bigger to begin with.

    Sorry if this just too basic, but it seems to answer your question.

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    Quote Originally Posted by finefocus View Post
    Subtract from +1.75 --- BUT --- don't say add or subtract, say combine. Combine two plus numbers, get a bigger number. Combine two minus numbers, get a bigger number (it's minus, but still bigger). Combine a plus and a minus (like your example) and get a smaller number. Will it be plus or minus? Depends on which was bigger to begin with.

    Sorry if this just too basic, but it seems to answer your question.
    Subtract your answer from the sphere power

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    Quote Originally Posted by Lensman11 View Post
    Subtract your answer from the sphere power
    Actually, you would add it; it's just that you're adding a negative to a positive. Subtracting a negative is the same as adding a positive. That's why add/subtract can confuse people.

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    Quote Originally Posted by finefocus View Post
    Actually, you would add it; it's just that you're adding a negative to a positive. Subtracting a negative is the same as adding a positive. That's why add/subtract can confuse people.
    Actually you always add it algebraic you but she she said her math skills were lacking so a simple answer should suffice.

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