Question

A soma dos 20 primeiros da pa (1/2,3/2...,)

Answer 1

$r = a2 - a1 = \frac{3}{2} - \frac{1}{2} = \frac{2}{2} = 1 \\ a20 = a1 + 19r \\ a20 = \frac{1}{2} + 19.(1) \\ a20 = \frac{1}{2} + 19 \\ a20 = \frac{1 + 38}{2} \\ a20 = \frac{39}{2} \\ \\ s20 = \frac{(a1 + a20).n}{2} \\ s20 = \frac{( \frac{1}{2} + \frac{39}{2}).20 }{2} \\ sn = \frac{ (\frac{40}{2}).20 }{2} \\ sn = \frac{ \frac{800}{2} }{2} \\ sn = \frac{800}{2} \times \frac{2}{1} \\ sn = \frac{1600}{2} \\ sn = 800$