$$A=2.3+3.4+4.5+\ldots +17.18$$
Aradığımız toplama $B$ diyelim.
$$B=2.2+3.3+4.4+\ldots +17.17=2.(3-1)+3.(4-1)+4.(5-1)+\ldots +17.(18-1)$$
$$\Rightarrow$$
$$B=2.3+3.4+4.5+\ldots +17.18 -(1+2+3+\ldots +17)$$
$$\Rightarrow$$
$$B=A-\frac{17.18}{2}=A-17.9$$