Câu 10 :
a ) A = 1 + 3 + 32 + 33 +...+3100
3A = 3(1+3+32+33+....+3100 )
3A = 3 + 32 +33 +34 +... +3101
3A - A = 2A = (3+32+33+34+...+3101) - (1 + 3 + 32+33+....+3100)
2A = ( 3 - 3 ) + (32-32) + (33 + 33 ) +....+(3100 -3100 ) + (3101 - 1 )
2A = 0 + 0 + 0 +....+0 + 3101 - 1
2A = 3101-1
A = (3101 - 1 ) : 2
b)B = 1 + 4 +42 + 43 +....+ 4100
4B = 4(1 + 4 + 42 + 43 +....+4100)
4B = 4 + 42+43+44+....+4101
4B-B = 3B = ( 4+42+43+44+....+4101) - ( 1 + 4+42+43+....+4100)
3B = (4-4 ) + (42-42) + (43-43)+....+ (4100 - 4100 ) + 4101 - 1
3B= 4101 - 1
B = (4101 - 1 ) : 3
d.D = 3100 + 3101 + 3102 +....+3150
3D = 3(3100 + 3101 + 3102 + ....+3150)
2D = ( 3101 +3102 +3103 +.....+3151) - ( 3100 + 3101 + 3102+....+3150)
2D = 3151 - 3100
D = (3151 - 3100 ) : 2
Còn một bước nữa nhé
a. 3A = (3101 - 1 ) : 2 . 3
b.4B = (4101 - 1 ) : 3 . 4
d.3D = (3151 - 3100 ) : 2 . 3
