Rules of calculus are tools that make complicated calculations easy. Just as you use addition, subtraction, multiplication and division for ordinary arithmetic, calculus is used in a similar way. Newton is given the credit for the discovery of calculus. Almost all branches of engineering mathematics use calculus. If you know an equation that is generating the geometric shape of the object, then calculus can help you calculate area, volume, slopes at different points, etc.
Application of calculus are enormous.
Q. Infinity is a number which is either too large or too small to compute. For example, if you divide 1 by 1000, you get 0.001. Continue this process just to see for yourself the result. Divide 1 by 1000000, you get 0.000001. Hence division of any number by infinite number is zero.