Math Prep

kun861010

if x is equal to the sum of the even integers from 40 to 60, inclusive, and y is the number of even integers from 40 to 60 inclusive, what is the value of x+y?
a,550
b,551
c,560
d,561
e,572
答案是d，请问这个561怎么来的。。。。怎么会有1呢？

rannie_zhao

因为40到60间的偶数有11个，即y=11
40到60间的偶数和是（40+60）11/2=550，即x=550
x+y=550+11=561