[Toán đại 8] Tính giá trị biếu thức

K

kool_boy_98

$x^{15} –8x^{14} +8x^{13} –8x^{12}+... – 8x^2 +8x –5$
$= x^{15} –8.(x^{14} –x^{13} +x^{12}–... + x^2 –x+1) +3$
$= x^{15} –\frac{8}{x +1}. (x+1)(x^{14} –x^{13} +x^{12}–... + x^2 –x+1) +3$
$= x^{15} –\frac{8}{x +1}. (x^{15} +1) +3 $
Thay $x = 7$ ta được:
$x^{15} –\frac{8}{x +1}. (x^{15} +1) +3$
$= x^{15} –(x^{15}+1) +3$
$= 2$

[Sưu tầm]
 
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