To prove that the sum \( \frac{3}{4} + \sqrt{10} \) is irrational, we will use a proof by contradiction.
Assume that the sum \( x = \frac{3}{4} + \sqrt{10} \) is rational. By definition, a rational number can be expressed as the ratio of two integers \( p/q \) where \( p \) and \( q \) are integers and \( q \neq 0 \).
From our assumption:
\[
\sqrt{10} = x - \frac{3}{4}.
\]
Since \( x \) is rational and \( \frac{3}{4} \) is rational, the right-hand side \( x - \frac{3}{4} \) is also rational. This means that \( \sqrt{10} \) is rational as well.
However, \( \sqrt{10} \) is known to be an irrational number since it cannot be expressed as a fraction of two integers. Thus, we reach a contradiction where we assumed that a rational number led to the conclusion that \( \sqrt{10} \) is rational. Therefore, our original assumption that \( \frac{3}{4} + \sqrt{10} \) is rational must be false.
Thus, the sum \( \frac{3}{4} + \sqrt{10} \) is irrational.
In summary, the sum is irrational because assuming it to be rational leads to the false conclusion that \( \sqrt{10} \) is rational, which contradicts the established knowledge that \( \sqrt{10} \) is irrational.