Proof: For the given statement, the property P(n) is the
equation
2 + 4 + 6+· · ·+2n = n2 + n. ← P(n)
Show that P(1) is true:
To prove P(1), we must show that when 1 is substituted
into the equation in place of n, the left-hand side equals
the right-hand side. But when 1 is substituted for n, the
left-hand side is the sum of all the even integers from 2
to 2· 1, which is just 2, and the right-hand side is 12 + 1,
which also equals 2. Thus P(1) is true.
Show that for all integers k ≥ 1, if P(k) is true then P(k+1)
is true:
Let k be any integer with k ≥ 1, and suppose P(k) is true.
That is, suppose
2 + 4 + 6+· · ·+2k = k2 + k. ← P(k)
inductive hypothesis
We must show that P(k + 1) is true. That is, we must show
that
2 + 4 + 6+· · ·+2(k + 1) = (k + 1)2 + (k + 1).
Because (k + 1)2 + (k + 1) = k2 + 2k + 1 + k + 1 =
k2 + 3k + 2, this is equivalent to showing that
2 + 4 + 6+· · ·+2(k + 1) = k2 + 3k + 2. ← P(k + 1)
But the left-hand side of P(k + 1) is
2 + 4 + 6+· · ·+2(k + 1)
= 2 + 4 + 6+· · ·+2k + 2(k + 1)
by making the next-to-last
term explicit
= (k2 + k) + 2(k + 1) by substitution from the
inductive hypothesis
= k2 + 3k + 2, by algebra,
and this is the right-hand side of P(k + 1). Hence P(k + 1)
is true.
[Since both the basis step and the inductive step have been
proved, P(n) is true for all integers n ≥ 1.]