Question

Using the rule, can’t you do these in your head?

(i) (−3) × 4

(ii) (−4) × 7

(iii) (−1) × 9

(iv) (−5) × 4

(v) (−2) × 5

(vi)

(vii)

(viii)

(ix)

(x) (−0.5) × 0.2

(xi) (−0.4) × 0.3

Answer 1

(i)

(−3) × 4 = −(3 × 4) {(−x) × y = −(xy), for all positive numbers x and y}

= −12

(ii)

(−4) × 7 = −(4 × 7) {(−x) × y = −(xy), for all positive numbers x and y}

= −28

(iii)

(−1) × 9 = −(1 × 9) {(−x) × y = −(xy), for all positive numbers x and y}

= −9

(iv)

(−5) × 4 = −(5 × 4) {(−x) × y = −(xy), for all positive numbers x and y}

= −20

(v)

(−2) × 5 = −(2 × 5) {(−x) × y = −(xy), for all positive numbers x and y}

= −10

(vi)

× = − {(−x) × y = −(xy), for all positive numbers x and y}

= −1

(vii)

× = {(−x) × y = −(xy), for all positive numbers x and y}

=

= −

(viii)

× = {(−x) × y = −(xy), for all positive numbers x and y}

=

= −

(ix)

× = {(−x) × y = −(xy), for all positive numbers x and y}

=

= −

(x)

(−0.5) × 0.2 = −(0.5 × 0.2) {(−x) × y = −(xy), for all positive numbers x and y}

= −0.1

(xi)

(−0.4) × 0.3 = −(0.4 × 0.3) {(−x) × y = −(xy), for all positive numbers x and y}

= −0.12