a) 13 . 65 + 35 . 12
= 13 . 13 . 5 + 5 . 7 . 12
= 169 . 5 + 5 . 84
= 5 . (169 + 84)
= 5 . 253
= 1265
b) 31 + 32 + 33 + ... + 39 - 21 - 22 - ... - 29
= (31 - 21) + (32 - 22) + (33 - 23) + (34 - 24) + (35 - 25) + ... + (39 - 29)
= 10 + 10 + 10 + 10 + ... + 10 (chín số 10)
= 9 . 10
= 90
c) 3 - 8 . 27 + 4 . 33 . 6 + 40 . 2 . 12
= 3 - 8 . 9 . 3 + 4 . 33 . 2 . 3 + 40 . 2 . 4 .3
= 3 - 72 . 3 + 264 . 3 + 320 . 3
= 3 . (1 - 72 + 264 + 320)
= 3 . 513
= 1539
