You can easily do the square of two digit number starting with 5 using this rule.
Suppose you have a number of the format 5x to be square for example say 54. Then it can be formed mentally using as follows:
- First do X + 25 – then in our example it will be 4 + 25 = 29
- The do the X2 – In our example it will 42 = 16
- Assemble the two results as follows – 2916
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