Bài 3:
a, $x+(x+1)+(x+2)+...+(x+99)=5450$
$⇒100x+(1+2+...+99)=5450$
$⇒100x+(99.49+99)=5450$ (tổng 1+2+..+99 thửa 1 số hạng nên mình nhóm riêng ra nha)
$⇒100x+4950=5450$
$⇒100x=500$
$⇒x=5$
b, $3.(5^x-1)-2=70$
$⇒3.(5^x-1)=72$
$⇒5^x-1=24$
$⇒5^x=25$
$⇒5^x=5^2$
$⇒x=2$
c, $2^x+2^{x+1}+2^{x+2}=960-2^{x+3}$
$⇒2^x+2^{x+1}+2^{x+2}+2^{x+3}=960$
$⇒2^x.(1+2+2^2+2^3)=960$
$⇒2^x.15=960$
$⇒2^x=64$
$2^x=2^6$
$⇒x=6$
