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

1. ## 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....

2. Originally Posted by Lori
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. at what step do i subtract it; subtract it from what? sorry, not a math wiz.

4. Originally Posted by Lensman11
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.

5. Originally Posted by Lensman11
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...

6. Originally Posted by jefe
+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.

7. Originally Posted by Lori
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.

8. Originally Posted by finefocus
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.

9. Originally Posted by Lensman11
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.

10. Originally Posted by finefocus
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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