Solve the equation:
3√x2+4=√x−1+2x−3
Things I have done so far:
3√x2+4=√x−1+2x−3 Deducting 2 from both sides of equation
⇔(3√x2+4−2)=(√x−1−1)+2x−4
⇔x2+4−83√(x2+4)2+23√x2+4+4=x−2√x−1+1+2(x−2)
⇔(x−2)(x+23√(x2+4)2+23√x2+4+4−1√x−1+1−2)=0
+)x−2=0⇔x=2
+)x+23√(x2+4)2+23√x2+4+4−1√x−1+1−2=0(∗)
I don't know how to solve the equation (*).
By the way, I think there must be "smart" way to solve this equation.
Answer
3√x2+4=√x−1+2x−3
Let x=t2+1 with t≥0. Then
3√t4+2t2+5−t=2t2−13√t4+2t2+5=2t2+t−1t4+2t2+5=8t6+12t5−6t4−11t3+3t2+3t−18t6+12t5−7t4−11t3+t2+3t−6=0(8t5+20t4+13t3+2t2+3t+6)(t−1)=0
Note, for t≥0, that 8t5+20t4+13t3+2t2+3t+6>0. It follows that the only positive solution is t=1. Hence x=2.
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