Squaring Numbers ending in 9

Squaring Numbers ending in 9
To square numbers ending in 9 like 19, 29, 39, 49....you know square of 9 is 9 x 9= 81, you can use this for speedy calculation. Now to the technique let me explain with example

1^st example 19²

step 1 : Add 1 to the number so that the number will be ending in 0
             19+1 =20
step 2 : find square of it as it ends in zero finding its square is easy.
              20²  = 400
step 3 : Add the 19 to 20
              19 + 20 = 39
step 4 : subtract the result of step 3 from resutl of step 2 and that will be your answer
              400 - 39= 361
              thus 19²  = 361

2^nd example 79²

step1 : 79+1 =80

step2 : 80²  = 6400

step3 : 79+80 =159

step4 : 6400-159=6241

thus 79²  = 6241