4 sau 6
Dau coroana la cel care răspunde corect

4)

a)
1+2+3+...+30
=30×31:2=
=930:2
=465

b)
2+3+4+...+30
=30×31:2-1=
=930:2-1
=465-1=
=464

c)
9+10+11+...+30
=1+2+3+...+8+9+10+11+...+30-(1+2+3+...+8)=
=30×31:2-8×9:2=
=930:2-72:2=
=465-36=
=429


6)

a) 7ab+ab2=977

7+a=9
a=9-7
a=2

b+2=7
b=7-2
b=5

Proba :
725+252=977



b)

4ab+ba9=852

b+9=12
b=12-4
b=3

a+a+1=15
2a+1=15
2a=14
a=7

Proba :
473+379=852