Page:Elementary algebra (1896).djvu/189

INVOLUTION. 171 Rule II. The square of the difference of two quantities is equal to the sum of their squares diminished by twice their product.

189. These rules may sometimes be conveniently applied to find the squares of numerical quantities.

TO SQUARE A MULTINOMIAL.

190. We may now extend the rules of Art. 188 thus:

In the same way we may prove In each instance we observe that the square consists of

(1) the sum of the squares of the several terms of the given expression;

(2) twice the sum of the products two and two of the several terms, taken with their proper signs; that is, in each