Shortcut Trick
Identify the signs of the cross-product terms in the expression: −6ab, +6bc, and −2ac.
The term +6bc is positive, which implies that b and c must have the same sign.
The terms −6ab and −2ac are negative, which implies that a must have the opposite sign of b and c.
Looking at the options, only (a − 3b − c)2 satisfies these conditions (positive a, negative b and c).
∴ The correct answer is (a − 3b − c)2.
Alternate Method
Given Expression: a2 + 9b2 + c2 − 6ab + 6bc − 2ac
Formula Used: (x + y + z)2 = x2 + y2 + z2 + 2xy + 2yz + 2zx
We need to represent the given terms as squares and pairs:
⇒ x2 = a2 → x = a
⇒ y2 = 9b2 = (−3b)2 → y = −3b (chosen negative to match the −6ab term)
⇒ z2 = c2 = (−c)2 → z = −c (chosen negative to match the −2ac and +6bc terms)
Now, verify the cross-products:
⇒ 2xy = 2 × a × (−3b) = −6ab
⇒ 2yz = 2 × (−3b) × (−c) = +6bc
⇒ 2zx = 2 × a × (−c) = −2ac
All terms match the original expression, so:
⇒ a2 + 9b2 + c2 − 6ab + 6bc − 2ac = (a + (−3b) + (−c))2
⇒ (a − 3b − c)2
∴ The correct answer is (a − 3b − c)2.
Additional Information
Square of a Trinomial
The general expansion is (a + b + c)2 = a2 + b2 + c2 + 2ab + 2bc + 2ca. The sign of the squared terms is always positive for real numbers.
Sign Convention
When one or more variables are negative, the sign of the product terms (ab, bc, ca) changes accordingly. For example, (a − b + c)2 = a2 + b2 + c2 − 2ab − 2bc + 2ca.
Perfect Square Identification
To identify a perfect square trinomial, check if the square root of the squared terms, when multiplied by 2 in pairs, matches the remaining terms of the expression.