That's not correct. The terms 2a and 3b are not like terms, so we cannot combine them to get 5ab. We simply leave it as 2a+3b.
If you had 2a+3a, then it would simplify to 5a
Similarly, 2b+3b = 5b
Or you could have 2ab+3ab = 5ab
The key is that the variable portions must match up to be able to add them.
Part (c)
We'll use this identity

to say

Similarly,

-------------------------
The key takeaways here are that

Therefore,

The identity is confirmed.
==========================================================
Part (d)

Similarly,

-----------------
We'll square each equation

and

--------------------
Let's compare the results we got.

Now if we add the terms straight down, we end up with
on the left side
As for the right side, the sin(A)cos(A) terms cancel out since they add to 0.
Also note how
and similarly for the sin^2 terms as well.
The right hand side becomes
but that's always equal to 1 (pythagorean trig identity)
This confirms that
is an identity
Answer:
The claim that the current work teams can build room additions quicker than the time allotted for by the contract has strong statistical evidence.
Step-by-step explanation:
We have to test the hypothesis to prove the claim that the work team can build room additions quicker than the time allotted for by the contract.
The null hypothesis is that the real time used is equal to the contract time. The alternative hypothesis is that the real time is less thant the allotted for by the contract.

The significance level, as a storng evidence is needed, is α=0.01.
The estimated standard deviation is:

As the standard deviation is estimated, we use the t-statistic with (n-1)=15 degrees of freedom.
For a significance level of 0.01, right-tailed test, the critical value of t is t=2.603.
Then, we calculate the t-value for this sample:

As the t-statistic lies in the rejection region, the null hypothesis is rejected. The claim that the current work teams can build room additions quicker than the time allotted for by the contract has strong statistical evidence.