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thanks for the answer ... but my actual doubt is .. i have casio fx-991ms calculator .. how to use 'i' in that ?   i pressed shift and i..... nothing comes ,  i too pressed alpha and i ... nothing appears ..??? any idea?

I¹

ī/ noun

1.the ninth letter of the alphabet.

2.the Roman numeral for one.

I is used to represent your self:

"I just bought some food from Walmart."

This represents yourself buying food from Walmart.

I think he means... i as in the imaginary number.

First off you have to know that i = sqrt(-1)

Say if you have sqrt(-6):

Since there cannot be a negative rooted to any EVEN power then there has to be an i in the simplified form.

You would end up getting i * sqrt(6)

Iḿ sure you can tell what I mean at this point. 

If you want to use i on the calculator on this site, just type in "i"

On a graphing calculator such as a TI-84, there should be an i key in "shift" mode.

Any other questions?! Just ask!

Umm... Casio's are not my strongpoint. Try to look for it in the catalog if your calculator has it. l'm not sure if Casio's even support imaginary numbers.