a) A = (m – n)² + 4mn
⇔ A = m² - 2mn + n² + 4mn
⇔ A = m² + 2mn + n²
⇔ A = (m + n)²
b) B = (6z – 2)² + 4(3z - 1)(2 + t) + (t + 2)²
⇔ B = 36z² - 24z + 4 + 4(6z - 2 + 3zt - t) + t² + 4t + 4
⇔ B = 36z² - 24z + 4 + 24z - 8 + 12zt - 4t + t² + 4t + 4
⇔ B = 36z² + ( - 24z + 24z) + ( -4t + 4t) + 12zt + (4 + 4 - 8) + t²
⇔ B = (6z)² + 2.6zt + t²
⇔ B = (6z + t)²