In scientific notation, we write a number so that it has single digit to the left of decimal sign and is multiplied by an integer power of #10#.

Note that moving decimal #p# digits to right is equivalent to multiplying by #10^p# and moving decimal #q# digits to left is equivalent to dividing by #10^q#.

Hence, we should either divide the number by #10^p# i.e. multiply by #10^(-p)# (if moving decimal to right) or multiply the number by #10^q# (if moving decimal to left).

In other words, it is written as #axx10^n#, where #1<=a<10# and #n# is an integer.

To write #0,0045# in scientific notation, we will have to move the decimal point three point to right, which literally means multiplying by #1000=10^3#.

Hence in scientific notation #0.0045=4.5xx10^(-3)# (note that as we have moved decimal three point to right we are multiplying by #10^(-3)#.