①
4(m+n)
4 m 2 +8mn+4 n 2 =
4(m+n)
4(m+n ) 2 =
1
m+n ,计算正确;
②
x+y+1
-x+y+1 =-
x+y+1
x-y-1 ,不能约分,计算错误;
③
m 2 -3m+2
m- m 2 =-
(m-1)(m-2)
m(m-1) =
2-m
m ,计算正确;
④(a+b)÷(a+b)•
1
a+b =(a+b)•
1
a+b •
1
a+b =
1
a+b ,计算错误;
计算正确的有两个,故选B.
①
4(m+n)
4 m 2 +8mn+4 n 2 =
4(m+n)
4(m+n ) 2 =
1
m+n ,计算正确;
②
x+y+1
-x+y+1 =-
x+y+1
x-y-1 ,不能约分,计算错误;
③
m 2 -3m+2
m- m 2 =-
(m-1)(m-2)
m(m-1) =
2-m
m ,计算正确;
④(a+b)÷(a+b)•
1
a+b =(a+b)•
1
a+b •
1
a+b =
1
a+b ,计算错误;
计算正确的有两个,故选B.