Đáp án:
Bài 1:
a) $3^{x-1}+5.3^x=432$
$\dfrac{3^x}{3}+5.3^x=432$
$\left(\dfrac{1}{3}+5\right).3^x=432$
$\dfrac{16}{3}3^x=432$
$3^x=81=3^4$
$x=4$
b)
$(x+2)+(x+7)+(x+12)+...+(x+42)+(x+47)=945$
$(x+2)+(x+2+5)+(x+2+10)+....+(x+2+40)+(x+2+45)=945$
$10(x+2)+(5+10+...+40+45)=945$
$10(x+2)+125=945$
$10(x+2)=820$
$x=80$
c)
$2.3^x+3^{x+2}=99$
$2.3^x+3^2.3^x=99$
$11.3^x=99$
$3^x=9=3^2$
$x=2$