7^86+7^85-7^84
=7^84.7^2+7^84.7-7^84.1
=7^84.(7^2+7-1)=7^84.55 chia het cho 55
Tick nhé
\(7^{86}+7^{85}-7^{84}=7^{84}.7^2+7^{84}.7+7^{84}.1\)
\(=7^{84}.\left(49+7-1\right)=7^{84}.55=7^{84}.5.11\)
Chia het cho 55
786+785-784=784.72+784.7-784=784.(72+7-1)=784.55 chia hết cho 55
=>dpcm
Ta có:
786+785-784=784.(72+7-1)
=784.55
Vì 784.55 chia hết cho 55
=>786+785-784 chia hết cho 55 (đpcm)
786 + 785 - 784
= 784 . 72 + 784 . 7 - 784 . 1
= 784 . (72 + 7 - 1)
= 784 . (49 + 7 - 1)
= 784 . 55
Vì 55 chia hết cho 55 => 784 . 55 chia hết cho 55
Vậy 786 + 785 - 784 chia hết cho 55