Question

вычислите: 1+2-3+4+5-6+7+8-9+,,,+208+209-210

Answer 1

$a _{1} =1 \\ a _{2} =2 \\ d=2-1=1 \\ a _{210} =210 \\ \\ S= \frac{a _{1} +a _{210} }{2} *n= \frac{1+210}{2} *210= \frac{211}{2} *210= \\ \\ =22155$