Re: How many integer values of $n$ are there such that $8<-2 n^2+40<38$ ?
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12 May 2025, 02:43
First, let's split the compound inequality into two parts:
1. $\(8<-2 n^2+40\)$
2. $\(-2 n^2+40<38\)$
Part 1: Solve $\(8<-2 n^2+40\)$
$$
\(8<-2 n^2+40\)
$$
Subtract 40 from both sides:
$$
\(\begin{gathered}
8-40<-2 n^2 \\
-32<-2 n^2
\end{gathered}\)
$$
Divide both sides by -2 (remember to reverse the inequality sign when dividing by a negative number):
$$
\(\begin{aligned}
& 16>n^2 \\
& n^2<16
\end{aligned}\)
$$
This implies:
$$
\(-4<n<4\)
$$
Part 2: Solve $\(-2 n^2+40<38\)$
$$
\(-2 n^2+40<38\)
$$
Subtract 40 from both sides:
$$
\(-2 n^2<-2\)
$$
Divide both sides by -2 (reverse the inequality sign again):
$$
\(n^2>1\)
$$
This implies:
$$
\(n<-1 \quad \text { or } \quad n>1\)
$$
Step 2: Combine the Results
From Part 1, we have:
$$
\(-4<n<4\)
$$
From Part 2, we have:
$$
\(n<-1 \quad \text { or } \quad n>1\)
$$
Combining these, we get two ranges for $n$ :
1. $\(-4<n<-1\)$
2. $\(1<n<4\)$
Step 3: Find Integer Solutions
Now, let's list the integer values of $n$ that fall within these ranges.
Range 1: $\(-4<n<-1\)$
Possible integer values:
$$
\(n=-3,-2\)
$$
Range 2: $\(1<n<4\)$
Possible integer values:
$$
\(n=2,3\)
$$
Step 4: Verify the Solutions
Let's verify each potential solution in the original inequality to ensure they satisfy both parts.
1. For $\(n=-3\)$ :
$$
\(-2(-3)^2+40=-2(9)+40=-18+40=22
$$\)
Check: $\(8<22<38 \rightarrow\)$ Valid
2. For $\(n=-2\)$ :
$$
\(-2(-2)^2+40=-2(4)+40=-8+40=32\)
$$
Check: $\(8<32<38 \rightarrow\)$ Valid
3. For $\(n=2\)$ :
$$
\(-2(2)^2+40=-2(4)+40=-8+40=32\)
$$
Check: $\(8<32<38 \rightarrow\)$ Valid
4. For $\(n=3\)$ :
$$
\(-2(3)^2+40=-2(9)+40=-18+40=22\)
$$
Check: $\(8<22<38 \rightarrow\)$ Valid
Step 5: Check Boundary Cases
The inequalities are strict (< and >), so we exclude the boundary values:
- $\(n=-4: n^2=16 \rightarrow\)$ Violates $\(n^2<16\)$
- $\(n=-1: n^2=1 \rightarrow\)$ Violates $\(n^2>1\)$
- $\(n=1: n^2=1 \rightarrow\)$ Violates $\(n^2>1\)$
- $\(n=4: n^2=16 \rightarrow\)$ Violates $\(n^2<16\)$
Conclusion
There are four integer values of $n$ that satisfy the inequality: $\(-3,-2,2,3\)$.