90+91+92+93+94+95+96+97+98+99
=(90+99)*10/2
=189*5
=945请问（90+99)*10/2这步是怎么算的？？90+91+92+93+94+95+96+97+98+99=(90+99)+(91+98)+(92+97)+(93+96)+(94+95)=(90+99)*10/2(10个数，5组）=189*5=945