11+13+15+17+19=(15)*(5)=(75 )
11+13+15+17+19+21=(16 )*(6 )=(96 )
3+6+9+······+99=(51 )*(33 )=(1683 )
1+4+7+······+100=(50.5 )*(34 )=(1717 )
101+103+105······+199=(150 )*(50)=(7500 )
取数列中间值乘以项数,即首尾相加的数字除以2然后乘以数列的个数.(只适用于等差数列)
11+13+15+17+19=(15)*(5)=(75 )
11+13+15+17+19+21=(16 )*(6 )=(96 )
3+6+9+······+99=(51 )*(33 )=(1683 )
1+4+7+······+100=(50.5 )*(34 )=(1717 )
101+103+105······+199=(150 )*(50)=(7500 )
取数列中间值乘以项数,即首尾相加的数字除以2然后乘以数列的个数.(只适用于等差数列)