In triangle ABC, the medians AD,BE, and CF concur at the centroid G.

(a) Prove that AD < (AB + AC)/2.
(b) Let P=AB+AC+BC be the perimeter of triangle ABC. Prove that 3P/4 < AD + BE + CF < P.

To prove the given inequalities, we need to use the properties of medians in triangles.

For (a) AD < (AB + AC)/2, we can use the fact that the medians of a triangle divide each other in the ratio 2:1. So, let's consider the median AD.

We know that the length of the median AD is two-thirds of the length of the side opposite it. In this case, AD is two-thirds of BC.

Now, let's label the length of BC as x. Therefore, AD = (2/3)x.

By triangle inequality, we know that in any triangle, the length of a side is always less than the sum of the other two sides.

So, in triangle ABC, we have: BC < AB + AC.

Substituting x for BC, we have: x < AB + AC.

Multiplying both sides by (2/3), we get: (2/3)x < (2/3)(AB + AC).

Since AD = (2/3)x, we have: AD < (2/3)(AB + AC).

Simplifying, we get: AD < (AB + AC)/3.

We need to prove that AD < (AB + AC)/2.

To do this, we can prove that (AB + AC)/3 < (AB + AC)/2.

Multiplying by 3 on both sides, we have: AB + AC < (3/2)(AB + AC).

Simplifying, we get: AB + AC < 3AB/2 + 3AC/2.

Subtracting AB + AC from both sides, we have: 0 < AB/2 + AC/2.

This inequality is true since AB and AC are positive lengths.

So, we have proven that AD < (AB + AC)/2.

Now, let's move on to (b) AD + BE + CF < P.

Again, using the property that the medians of a triangle divide each other in the ratio 2:1, we can say that AD + BE + CF is less than the sum of the three sides of the triangle, which is the perimeter P.

So, we have: AD + BE + CF < P.

Next, let's prove that 3P/4 < AD + BE + CF.

To do this, we can use the fact that the sum of the medians in a triangle is three-fourths of the perimeter.

So, we have: AD + BE + CF = (3/4)P.

Now, we need to prove that (3/4)P < AD + BE + CF.

To do this, we can prove that P < (4/3)(AD + BE + CF).

Multiplying by 3/4 on both sides, we have: (3/4)P < (3/4)(4/3)(AD + BE + CF).

Simplifying, we get: (3/4)P < AD + BE + CF.

Since we already established that AD + BE + CF < P, this inequality is true.

So, we have proven that 3P/4 < AD + BE + CF.

Therefore, both (a) and (b) have been proven.